Only use ampersands within DesktopUI (PLEASE READ BEFORE MERGING)

PLEASE DO NOT MERGE UNTIL THIS IS MERGED:
https://github.com/hrydgard/ppsspp/pull/3526

Thank you.

Anyway, this updates to put ampersands only in DesktopUI.  For clarity, some additional notes are added and the DesktopUI section has been organized for translation goodness.

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