GTK4 Port

This is the almost-finished GTK4 port. Please feel free to join in the development effort!

Before building, make sure that

• WITH_INTERNAL_2GEOM is on.
  • Either run cmake -DWITH_INTERNAL_2GEOM=ON path_to_your_source, or use the cmake gui to graphically edit CMakeCache.txt.
• The 3rdparty/2geom submodule is up to date.
  • Run git submodule update --init --recursive.
• Currently gtkmm >= 4.13.3 and glibmm >= 2.78.1 are needed both to build and for some critical fixes. However, if they are not installed, the build system is set up to build them automatically, so this should not be an issue.

To contribute to this MR, just push commits to the branch directly. New contributors can also open an MR targeting the branch gtk4.

If the branch needs rebasing onto master or the commit history needs otherwise cleaning up, please add a 🔧 reaction or something to the MR while you're doing it, to prevent someone else doing the same thing simultaneously. After updating the branch in this way, check if any new commits were added in the meantime and add them back. The rest of the time, just push normally, i.e. without --force.