meson-templates: improve gtkmm template

I rebased this old MR (!191 (closed)) onto current master and made some minor fixes. We should bring in all template MRs in order to make Gtk4 templates next.

Message from old MR:

I reworked the file templates for C++. Part of the code in the UI file now differs only in two lines, instead of a separate block for C++. Also UI file looks much better and consistent. Just like for other languages. Screenshots under spoiler.

How UI file look changed

Before:

After:

Much of changes I described in comments. Most importantly, now you don’t have to manually add each element in the class constructor. And only those elements that you want to change or use from the code.