Migrate to gtksourceview-4

[stokito]

Small PR with update of the dependency. Nothing really changed in code, the only thing is that we have to init the sourceview on the start. Here is a migration guide:
https://developer.gnome.org/gtksourceview/unstable/porting-guide-3-to-4.html

[onkarshinde]

I world prefer if this dependency was managed through a build time flag and with ifdef in code. Do you think you van update your pull request to do that?

[stokito]

In fact we don't need to do this. The gtksourceview-4 is already widely adopted because GNOME's gedit is using it. It is ported to windows etc. So no need to use an old package which eventually will be abandoned.
But since this is a dependency change then the minor version of GNUSim should be incremented

[onkarshinde]

We currently publish a Windows build which is created on a fedora virtual machine using prebuilt mingw packages in repository. I can not find any gtksourceview-4 mingw package in repository, unless I did a mistake with how I have setup my fedora VM. That makes it impossible for me to create a Windows build.

[stokito]

ok, it may be that gtksourceview-4 wasn't released for Windows yet. I thought it already done because I used gedit for Windows which looks like uses the gtksourceview-4 but I'm not sure.
Anyway IMHO making the dependency conditional on build makes little sense and instead we can just wait for gtksourceview-4 being ported to Windows and then merge the PR. Maybe we should ping the gtksourceview-4 maintainers and ask them to do the release

[stokito]

Hi @onkarshinde I see that the gtksourceview-4 is published for Windows https://packages.msys2.org/search?q=sourceview
Could you please check?