Increase minimum required gtk version

[szanni]

The library currently requires gtk => 3.10 which was released in 2013.

Various new APIs we actually need have only become available in later releases:

• gdk_seat_grab needs at least 3.20 to fix uiDateTimePicker on Wayland [Enhancement] Make DateTimePicker better on Wayland #248
• Various GdkMonitor related API have only been introduced in 3.22 which will be needed to implement Window positioning on Linux, Microsoft and OSX? #260

This essentially means we will need to upgrade to at least 3.20 to fix issues around Wayland. I have also been looking more into multi monitor support and 3.22 brings quite a few new APIs that I've been using for my POC. Maybe the same would be possible with the older API, but I personally don't feel like wasting my time on that. The old APIs have been marked as deprecated for over 7 years now.

While at it, I was wondering if upgrading to 3.24 might even be an option as even ancient Debian systems support 3.24. RHEL 7 and RHEL 8 (Cent OS) only seem to support 3.22 though.

Upgrading to 3.24 would give us two things: we would never need to upgrade the gtk3 version ever again and we could safely remove unix/future.c. The future stuff requires pango 1.38 and gtk 3.24 requires pango 1.41
. gtk 3.22 only requires pango 1.37.3 . It does seem like RHEL 7 (Cent OS 7) does ship with pango 1.42 though, which would still allow us to remove all the unix future code. Are there more outdated, yet maintained system than RHEL?

My suggestion would hence be: set the minimum gtk version to 3.22.