Use same strings in GSettings schema and UI

While not all strings are applicable to the UI, try to reduce translator
effort by reducing the delta.
Move some GApplication handlers to EtApplication
Refactor about dialog handling

Fetch the parent from EtApplication.
Split off main() to a separate source file
Remove last uses of deprecated GtkStock
Remove use of deprecated GtkImageMenuItem
Move tagging code to new subdirectory

Rearranging the tagging code will make it easier to split the code off
to be a separate library.
Fix format specifier in GIO C++ wrapper string