Temporarily remove {pl,eu} translations of Ctrl+

They are problematic: because corresponding "ctrl" strings _also_ needed
by wx aren't translated, this causes broken accelerators on OS X.

Proper fix will be part of the next translations update, for now just
fix the bug this way.
Add more accel strings for translaiton

Add "ctrl", "alt" and "shift", also used by wx in addition to the
already covered "Ctrl+", "Alt+" and "Shift+".
OS X: don't use system regex.h in wx build

wx's internal copy must be used, because it is Unicode aware and doesn't
require a (possibly lossy) conversion to char*.

Fixes #125.
Use std::regex instead of wxRegEx

Don't use wxRegEx for mostly platform independent code in errors
recognition in gettext output, replace with std::wregex as in other
parts of the code.

Fixes #125.