Add Gtk4 support
This adds support for Gtk 4
- pkg-config uses name "gtk4" (opposed to "gtk+-3.0")
- IM modules are not supported in Gtk 4, see gtk#1181 (closed), so there is nothing to setup there with Gtk 4.
This worked for me to create an app bundle for Xournal++. In the xournalpp.bundle file previously used for Gtk 3 I had to replace <gtk>gtk+-3.0</gtk>
by <gtk>gtk4</gtk>
, remove the immodules and themes data part and replace gtk30
with gtk40
for the translations.