Improve shortcuts of Flip, X-Flip to match new labels

Merged zoner requested to merge zoner/grafX2:fix-flip-x-flip into master

This sets the shortcut keys for the palette menu's "Flip" and "X-Flip" buttons to "f" and "i" respectively, with proper letter underlining. The reason why this was messed up is because "Flip" used to be called "Invert", with "I" as the first letter in the label, hence the "i" shortcut, and "X-Flip" used to be called "X-Invert", with v as a shortcut. "i" is the letter in "X-Flip" that is underlined where the "v" used to be, so it was confusing to see the "F" being underlined for "Flip" when the shortcut key remained "i", and then for X-Flip, the "i" letter was underlined, causing the user to think "i" is the shortcut for X-Flip, but it really just did "Invert", the normal flip. I hope this makes sense. Either way, this should resolve trac ticket #48

Edited by zoner