Use GApplication::startup

Move initialisation code to on_application_startup() to better fit with
how GApplication should be used:

https://live.gnome.org/HowDoI/GtkApplication
Updated German translation
Do not clear finalized scanner GtkListStores

The GtkListStores are finalized when the scanner window is destroyed.
Avoid empty file list NULL pointer dereference
Updated Spanish translation
Review image help pages
Map "illustration" in picture name to correct type
Improve undo tooltip capitalisation
Finish the format specifier help page
Update the keyboard shortcuts help page
Mark introduction help pages as candidate
Updated Polish translation
Updated Czech translation
Updated Spanish translation
Set default response ID on GtkDialogs

Set a default response ID, so that the affirmative action is the default
action for dialogs.
Use GOptionContext for --help output
Make application startup similar to GtkApplication

Call gtk_init() at the end of startup, to act similarly to
GtkApplication. Move startup handler to EtApplication. Move gettext
initialisation to main().