Add tab to new window created with <Shift><Ctrl>N

[jeremypw]

Fixes #814

• New tab added in Application new-window action handler
• New window follows size of current window
• New tab follows current working directory of current window

There is scope for improving consistency and DRYing with code handling new windows created by dragging tabs, from the dock and from the commandline but that is left for now.

Code potentially changing the window size by adding a tab was removed as it did not seem appropriate. The window size is already controlled by settings, the user and the window manager.

[danirabbit]

Nice fix!