Update development documentation

[SenlisO]

Added more content to the developer documentation. These are changes off the top of my head. Feel free to request other changes, or make the changes yourself.

[pep8speaks]

Hello @SenlisO! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file usr/share/system-installer/UI/main.py:

Line 37:1: E402 module level import not at top of file
Line 43:1: E303 too many blank lines (5)
Line 66:80: E501 line too long (80 > 79 characters)
Line 68:80: E501 line too long (82 > 79 characters)
Line 117:80: E501 line too long (80 > 79 characters)
Line 127:80: E501 line too long (80 > 79 characters)
Line 130:80: E501 line too long (80 > 79 characters)
Line 133:80: E501 line too long (83 > 79 characters)
Line 160:80: E501 line too long (98 > 79 characters)
Line 473:80: E501 line too long (84 > 79 characters)
Line 514:80: E501 line too long (80 > 79 characters)
Line 554:80: E501 line too long (89 > 79 characters)
Line 555:49: E128 continuation line under-indented for visual indent
Line 555:80: E501 line too long (82 > 79 characters)
Line 581:80: E501 line too long (84 > 79 characters)
Line 664:80: E501 line too long (85 > 79 characters)
Line 680:80: E501 line too long (80 > 79 characters)
Line 712:80: E501 line too long (80 > 79 characters)
Line 721:80: E501 line too long (91 > 79 characters)
Line 795:80: E501 line too long (85 > 79 characters)
Line 802:80: E501 line too long (81 > 79 characters)
Line 808:80: E501 line too long (90 > 79 characters)
Line 818:80: E501 line too long (98 > 79 characters)
Line 841:80: E501 line too long (85 > 79 characters)
Line 862:80: E501 line too long (96 > 79 characters)
Line 909:80: E501 line too long (92 > 79 characters)
Line 910:45: E128 continuation line under-indented for visual indent
Line 911:45: E126 continuation line over-indented for hanging indent
Line 911:80: E501 line too long (94 > 79 characters)
Line 957:5: E303 too many blank lines (2)
Line 980:80: E501 line too long (90 > 79 characters)
Line 998:80: E501 line too long (82 > 79 characters)
Line 1027:5: E303 too many blank lines (2)
Line 1060:5: E303 too many blank lines (2)
Line 1060:58: E251 unexpected spaces around keyword / parameter equals
Line 1060:60: E251 unexpected spaces around keyword / parameter equals
Line 1064:80: E501 line too long (88 > 79 characters)
Line 1088:5: E303 too many blank lines (2)
Line 1182:80: E501 line too long (83 > 79 characters)
Line 1192:80: E501 line too long (94 > 79 characters)
Line 1204:80: E501 line too long (83 > 79 characters)
Line 1206:80: E501 line too long (91 > 79 characters)
Line 1216:16: E126 continuation line over-indented for hanging indent
Line 1217:16: E126 continuation line over-indented for hanging indent
Line 1219:80: E501 line too long (94 > 79 characters)
Line 1231:80: E501 line too long (87 > 79 characters)
Line 1255:80: E501 line too long (86 > 79 characters)
Line 1264:80: E501 line too long (86 > 79 characters)
Line 1268:9: E122 continuation line missing indentation or outdented
Line 1268:80: E501 line too long (121 > 79 characters)
Line 1269:80: E501 line too long (129 > 79 characters)
Line 1282:80: E501 line too long (80 > 79 characters)
Line 1283:80: E501 line too long (154 > 79 characters)
Line 1321:80: E501 line too long (89 > 79 characters)
Line 1347:80: E501 line too long (81 > 79 characters)
Line 1431:80: E501 line too long (83 > 79 characters)
Line 1497:32: E128 continuation line under-indented for visual indent
Line 1497:32: W503 line break before binary operator
Line 1521:80: E501 line too long (93 > 79 characters)
Line 1621:5: E303 too many blank lines (2)
Line 1645:5: E303 too many blank lines (2)
Line 1656:13: E124 closing bracket does not match visual indentation
Line 1657:18: E124 closing bracket does not match visual indentation
Line 1658:23: E124 closing bracket does not match visual indentation
Line 1659:28: E124 closing bracket does not match visual indentation

[SenlisO]

Started correcting pylint warnings.

Also consolidated some label code in main.py.

I made some mistakes changing imports where the find and replace did not add subprocess the way I wanted to. The error was made in 9cfbf60, but was fixed in 2dfd870.

[Batcastle]

Looks good