help-browser: Rewrite without GtkAction

The initial attempt of this commit was to remove the GtkAction usage, but grew a bit wider than that. The following happened:

• The dialog became a proper GObject, rather than being a big chunk of static variables that were hopefully initialized before you used them.
• The dialog now uses GActions to implement actions, and converted some signal handlers to actions as well.
• The plug-in run procedure now uses GtkApplication. This is one hand necessary to be able to set accelerators for actions, but on the other hand is more future-proof, as GTK4 removes gtk_main()