Fix some focus issues with toolbar spin buttons.
Fix some focus issues with toolbar spin buttons.

This now behaves more like the Pinta 1.7 implementation. After the value has finished editing (either by typing or using the +/- buttons), focus is returned to the canvas so that the spin button doesn't intercept further key presses that are intended for the tools.

PintaProject/Pinta,PintaProject/Pinta,PintaProject/Pinta