Replace _() with wxGetTranslation() for non-literal strings

[0tkl]

Building Aegisub against wxWidgets trunk (or 3.3.0) has been failing since wxWidgets/wxWidgets@3dddaeb . The updated doc said:

the _() macro is defined to do nearly the same thing as wxGetTranslation(), with the exception that the argument to _() must be a string literal.

Speaking of 2417c1f. Before applying the fix, the COMMAND_GROUP macro will expanded like this :

[arch1t3cht]

The _ around the parameters to COMMAND_GROUP is probably there so that xgettext can detect the strings as translatable strings. To work around that, COMMAND_GROUP could be added to the lists of keywords in make_pot.sh