spinbutton: Connect swipe gesture to the text entry

spinbutton: Connect swipe gesture to the text entry

In the GTK3 days, the spin button was an entry, with buttons on
top, and the swipe gesture affected the input on the entry bits.
Now the spin button is a container, so this gesture in the capture
phase applies to all contained children (incl. buttons).

Attach this gesture to the entry itself, so the buttons are left
outside this business. The gesture is still in the capture phase
in order to prevent text selection/edition/etc to happen.

Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/4008