plugins/showpaint: set default shortcut

This effects is off by default, which means to instruct someone how to use it for debugging purposes, you need to both tell them to enable it, and then also teach them how to assign a shortcut for it.

Since it's off by default, there's no harm in setting a shortcut, which simplifies the enablement instructions. I've chosen Meta+Ctrl+Alt+P, which uses so many modifiers that it won't conflict with anything.