Use "PO translation file" as filetype description.

This is more understandable to most people than "GNU gettext catalog",
where none of the three words is understandable without knowing gettext.
Mark app as DPI aware in win32 manifest.
Make sure settings are written on app shutdown.

E.g.  on OS X, window sizes wouldn't be stored due to wxConfig being
deleted before the windows.
Remember window positions on OS X too.
Better restoration of multiple windows' positions.

If more windows would occupy the same place, offset their positions slightly.
Don't crash when entering trailing \ on OS X.

Fixes #581, #576.
Better message when reporting uncaught exceptions.
Xcode: move the "Set CFBundleVersion from git" step to the top.

It needs to be done before linking, otherwise the build numbers in the
create .app and the corresponding dSYM (produced during Archive build)
wouldn't match, causing problems with e.g.  symbolication or HockeyApp
uploads.