[WIP] PR: Provide an API for external plugins to load/save/create new files

[ccordoba12]

This PR provides a public API for external plugins to register their own functionality when the following buttons/actions are pressed in Spyder toolbar and its File menu:

• New file
• Load file
• Save file
• Save all
• Save as
• Recent files

Fixes #7794.

Temporal TODOs (just so I don't forget to address them):

• Save button is disabled when a file in the Editor is saved. It should be enabled/disabled in a per plugin basis.
• Rename PluginWidget to BasePluginWidget (update changelog about that).
• Add comments explaining what the signals in BasePluginWidget do.

[pep8speaks]

Hello @ccordoba12! Thanks for updating the PR.

• In the file spyder/app/mainwindow.py, following are the PEP8 issues :

Line 81:1: E402 module level import not at top of file
Line 159:1: E402 module level import not at top of file

• In the file spyder/plugins/editor/plugin.py, following are the PEP8 issues :

Line 914:17: E127 continuation line over-indented for visual indent

Comment last updated on March 02, 2019 at 14:48 Hours UTC

[jitseniesen]

@ccordoba12 What's the plan with this? How about I take over?

My intention is to do some more work on spyder-notebook as it seems there is quite some interest in it.

[ccordoba12]

Closing because this was superseded by #22564.