Stop crash from occurring when trying to open a folder without a git repo

When a folder than did not have a git repo in it and a repo had already
was open a crash would occur. This crash was caused by revViewDelegate
 trying to access the repo pointer after it had been deleted. To fix this
before deleting repo we set the itemdeletegate to null.
Split revView into its own class

Tidying up mainwindow class so create revView into its own class.

 * A helper function was added to Commit and _type was removed due to not using
it.
 * Most functionallity handling revView was shifted into mainwindowrevview.cpp.
 That functionality has been refracted to make it easier to work with.
Move filesChanged view handling into its own class

To continue with the refactor of the mainWindow class fileChangedView has
been created which handles showing the files.
Make tabs triangular instead of square

Looks so much nicer than square
Add style sheet to tabs

This change makes the tabs stand out a bit more and a bit less standard.