SpinButton to InkSpinButton transition

This is the first step to transition our internal SpinButton from Gtk::SpinButton to InkSpinButton used by new UI elements.

In this MR SpinButton and toolbars have been updated.

Select toolbar when there's space:

...and in a narrow window:

Text toolbar with icons in spin buttons (space limited):

Remaining items:

• MathSpinButton
• Dialogs using spin buttons