Add keybind for centering scratch window vertically (center horiztonally/vertically by default for scratch window)

[jtaala]

Fixes #912.

[glupi-borna]

This seems to work nicely! The only thing I'm noticing is that the 'center-horizontally' action now centers scratch windows on both axes. I'm not sure if that is intentional or not, but I'm not opposed to it working that way at all.

EDIT: Nvm, just saw your comment where you explained that it is intentional. Good stuff, one less keybind to remember! :)

[jtaala]

This seems to work nicely! The only thing I'm noticing is that the 'center-horizontally' action now centers scratch windows on both axes. I'm not sure if that is intentional or not, but I'm not opposed to it working that way at all.

I might change this though. It feels good but isn't quite "center horizontally" as the keybind suggests. I'll add another keybind specifically for center window (including non-tiled). Will probably leave it unmapped by default.