Some button names should be translated, for instance Up, Left and such.
At the same time, some other button names shouldn't be translated,
for reasons that might be less obvious. In 0146456af, I removed the
_trans markers for button names that never need to be translated
(such as A and B), but that isn't actually enough to ensure that
DolphinWX won't try to translate them anyway. This commit adds a bool
that explicitly tells the GUI whether a button name should be translated.
Otherwise we'll have problems like the GUI treating the button name "B"
(which isn't supposed to be translated) as matching the translatable
string "B" (being an abbreviation of "bytes"), meaning that the button
"B" will be labeled "o" when running Dolphin in French (after
translations get pulled from Transifex the next time).
By the way, while it turned out that DolphinWX translated all button
names, it also turned out that DolphinQt2 translated *no* button names.
Go figure. This commit makes them consistent with each other.
