Add painting editor shortcuts

[jkulawik]

Having used the plugin for a while, I think the user experience would be greatly improved by adding the following shortcuts:

• Switch Paint/Fill/Erase modes (existing editor shortcuts might be an issue, so perhaps something like Shift+Q/W/E)
• Increase/decrease brush radius - many graphical programs use [ and ]
• Increase/decrease density - perhaps the same as the above but with Shift or control, or perhaps < and > (which technically are actually , and . )
• Perhaps the other 4 sliders, but I don't find myself changing those much