Convert TalkatuToolbar to GTK4

This is rather straightforward, because it simply involves replacing a toolbar with a box using a 'toolbar' style, and then running conversion on the `.ui` file.

Testing Done:
Compiled only the `talkatutoolbar.c.o` which passed. Also ran the `.ui` file through the GTK4 previewer.

Reviewed at https://reviews.imfreedom.org/r/1233/