Translate menu accelerators from XRC

The content of <accel> property was taken verbatim and appended to the
(translated) label; this bypassed wx's internal accelerators translation
mechanism, because wxMenuItem code quite reasonably assumes that the
string passed to it is translated.

Explicitly use SetAccel() instead, to force translation. This matters
for languages such as German where e.g. Ctrl+ is translated as Strg+.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78183 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Extract translator comments into wxstd.pot

Extract the commonly used TRANSLATORS: prefix into translation files, to
provide some additional context where needed.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78184 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Improve wxAcceleratorEntry::ToString() formatting

This method used the wxKeyNames[] array originally intended for parsing
accelerator specification and not for human-friendly formatting. In
particular, non-alphanumeric keys such as WXK_DOWN and many others were
formatted using all-uppercase name, which was highly unusual and not
native on any platforms (the only unaffected one was OS X).

Improve the formatting by doing the following:

a) use Title case for the entries; because parsing is case-insensitive,
   this has no effect on it;
b) add display_name field for keys where the symbolic name wasn't
   appropriate for display and set it for some of the keys that are
   likely to show up in menus;
c) add explanatory comments for translators

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78185 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Refresh PO files after accelerators changes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78186 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775