app: extract GtkApplicationAccels private class

app: extract GtkApplicationAccels private class

This has several benefits:
- Less code in GtkApplication. The accels handling is something
  self-contained, and GtkApplication now delegates the work.
- For the accels functions, there is now a distinction between static
  functions and functions in the gtkapplicationaccelsprivate.h header,
  which makes the code easier to understand, because we have a good
  overview just by reading the header.
- The struct _GtkApplicationPrivate is now easier to find instead of
  being in the middle of the file.

https://bugzilla.gnome.org/show_bug.cgi?id=764879