Handle (not) installed GTK+-Version

[infeo]

If GTK+-2.0 is not installed on the system, the packaged app refuses to start because it is build with passing jdk.gtk.version=2 to the JVM: https://github.com/cryptomator/cryptomator-linux/blob/5a5da37085eaaa57599158fba3ee7a3c5aa00a7a/.github/workflows/appimage.yml#L56

The current workaround is to unpack the resulting AppImage and edit the Cryptomator.cfg file, see https://github.com/cryptomator/cryptomator/issues/1167#issuecomment-625097731.

But the flag cannot be just removed, resulting application start failure on other systems, see https://github.com/cryptomator/cryptomator-linux/issues/9#issuecomment-624107896.

[infeo]

Blocked by #25

The jpackage apporach is not flexible enough, we need a shell script again.

[overheadhunter]

Blocked by #25

Still we won't start #25 until we know for sure there is a way to determine via shell if GTK 2 or 3 should be used.

[infeo]

The usage depends on if the appropiate gtk version is installed.

One way is to check is using the package manger. There are not so many, so for me this seems a feasible approach. With commands for dpkg and rpm we would already cover many major distributions (except ArchLinux):

• for Debian based distros

  user@ubuntu:~$ dpkg -l libgtk* | grep -e '^i' | grep -e 'libgtk-*[0-9]'
  ii  libgtk-3-0:amd64           3.24.18-1ubuntu1 amd64        GTK graphical user interface library
  ii  libgtk-3-bin               3.24.18-1ubuntu1 amd64        programs for the GTK graphical user interface library
  ii  libgtk-3-common            3.24.18-1ubuntu1 all          common files for the GTK graphical user interface library
  ii  libgtk2.0-0:amd64          2.24.32-4ubuntu4 amd64        GTK graphical user interface library - old version
  ii  libgtk2.0-bin              2.24.32-4ubuntu4 amd64        programs for the GTK graphical user interface library
  ii  libgtk2.0-common           2.24.32-4ubuntu4 all          common files for the GTK graphical user interface library
  ii  libgtk3-perl               0.037-1          all          Perl bindings for the GTK+ graphical user interface library

• for Fedora, CentOS, OpenSuse, etc

  [user@localhost ~]$rpm -qa | grep -e '^gtk[0-9]-[0-9][0-9]*'
  gtk3-3.24.20-1.fc32.x86_64
  gtk2-2.24.32-7.fc32.x86_64

I would say it is impossible to determine it for sure and use the flag only when we are able to detect that GTK+-2.x is installed.