Fix Eraser not loading upon first launching Krita

This is a mix of:

• The paintopbox not receiving the change of compositeop.
• Qt messing up with Krita's highlightable buttons.
• The paintopbox loading the preset before setCurrentView, which borks Qt from emitting the relevant update signals.

BUG: 393851
Differential Revision: https://phabricator.kde.org/D12832