SVN commit dialog now supports diff, add and revert actions.

Summary:
Added SVN commit dialog actions for diff file, add file and revert file.
This is done by commit dialog context menu actions.
SVN commit dialog is now a separate class and moved to a separate file.
Also added SVNCommands class with static methods to access basic SVN actions like geting revision or geting remote URL path for a file.
Also added ItemVersion::MissingVersion control for SVN plugin.

Test Plan: Run SVN commit dialog and check diff file works, revert works and add works.

Reviewers: VDG, elvisangelaccio, meven, ngraham

Reviewed By: VDG, meven, ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D27641