Add filterbox in Staged area (just like in Unstaged area)

[goran-w]

Add a searchfilter-box at the top of the Staged area under Local Changes, just like the one at the top of the Unstaged area.

This makes for a more symmetric and flexible handling of Staged/Unstaged:

• We should not have to Unstage All just to be able to apply a filter to the list.
• The Unstage All button should benefit from filtering, just like the Stage All button does.

NOTE - If a Staged-filter is active, a Commit could include more changes than the ones visible in the Staged area. To avoid such surprises (whenever a Staged-filter is active) we could either:

• Simply disallow committing (via App.RaiseException() warning), or
• Ask the user to confirm committing despite active Stage-filter (similar to ConfirmCommitWithoutFiles()).

[love-linger]

Done.