[Themes] Allow styling "added" files

[AmjadHD]

Problem description

"Added" files (i.e staged untracked files) do not have their own class or attribute to be styled differently then say staged modified files.

Preferred solution

Add a class "added" (or "created") to "file_diff_control staged" and add "icon_created" to added files controls.

Alternatives

N/A.

Screenshots

The first tab is modified and the third is added, but they cannot be distinguished from each other.