add settings to center buttons-grid

[rtgiskard]

buttonsGrid is using flowbox as the container, with default halign set to FILL, for which users have to tweak the button size to make it really fill the space or it will not be centered.

This adds options to make the buttonGrid centered properly

[rtgiskard]

Would be great if the column min/max wouldn't be needed, but that's sadly just how GTK works...

Not sure how one would fix this without needing those config properties. Any ideas?

Maybe use some other container other than flowbox. Anyway, for flowbox, align to center seems to require set_min_children_per_line() to be set properly.

[ErikReider]

For future reference: Here's something that would work as a replacement https://gitlab.gnome.org/GNOME/libadwaita/-/merge_requests/219