(bug 45540) Add tooltips to the proofread and edit buttons

This adds on-hover tooltips to the proofread and edit buttons
in proofread mode.

This doesn't yet handle the "first proofreading" scenario.

Change-Id: I5c962cb983f5ffe3625748bd985850f29579c946
Merge "(bug 45540) Add tooltips to the proofread and edit buttons"