Add a triangle to the popup button view

bug:7442434
Change-Id: I7b784f2a5662ef694daef6cd926f60694b3eee4f
Gallery2: Localize strings

Undo and redo were hardcoded in the photo editor. This allows their
localization.

Patch Set 2: Use new strings so that it may be better translated for
certain languages. (Translations in separate patch)

Change-Id: I63df977696b74128ca5e6fc45037b6b43d43a465