WIP: Stage selection improvements
There're two levels in commit activity to stage and unstage changes. The file level and the line level.
As the buttons to interact with changes are at the bottom of the source view area, makes sense that those buttons were aware of those two levels
- Change text of buttons "Stage/Unstage" "files/lines" if there're lines selected.
- Inverse the action if Ctrl is pressed. "Stage/Unstage" all but selected "files/lines".
Allow to interact with lines using keyboard
- Ctrl+Shift+Up/Down Mark current focus line.
- Ctrl+Space to select lines (Ctrl+Enter is not available as now launch commit dialog).