Project plugin: use KMoreTools to implement the Git context menu

Visible changes:
- Move git items away from submenu but put them under a menu section
which reduces amount of submenus
- If one of the items is not installed it is placed in the "More"
submenu to give a hint that the app exist and might be helpful
- new: add gitg
- NEW: add "Git Cola (View History)" that opens the history browser for
the given file
- Menu items get icons when available, e.g. git-cola and gitg
- Menu items get caption and its translation from desktop file

Replacing old code results in less code for the project plugin.

REVIEW: 122374

DIGEST:

GUI: