Allow corner-tiling by quickly combining edge tiling shortcuts
Currently the only way for a user to invoke corner-tiling is to manually set shortcuts for them. This patch adds another option: invoke the existing shortcuts for edge tiling in a combined manner in quick succession.
For example, hitting Meta+Left and then Meta+Up within a two-second period will tile the active window into the top left corner. In practice you hold down the Meta key and then press Left then Up (or Up and then Left), and I think it feels very natural. Linux Mint's window manager has this feature and I always missed it when I left Mint for the KDE world.
Autotests for existing tiling shortcuts are adjusted to not break, and additional tests for the new tiling options are added.