Allow move position command to calculate from right and/or bottom edge

[gwhitney]

Unlike Xorg geometry specifications (e.g. 80x30-10-10) or CSS specifications (using right and bottom properties), prior to this PR SwayFX has no way to specify a window position as an offset to the right edge or bottom edge of its containing workspace. However, it is convenient to have such a mechanism for positioning a window in, say, the bottom right corner of the screen.

This PR adds optional left/right and top/bottom keywords before each dimension of the move position <dim> <dim> command. To preserve the behavior of existing config files/sway commands, omitted keywords on each dimension behave like left and top respectively.

Note that partial credit for this PR should go to @cizra, who filed a similar PR with sway itself. If the sway team shows interest in the somewhat refined version as filed here, we will likely file a very similar or identical PR to sway.

I wasn't sure if there were any other steps in generating a PR besides the code and documentation changes in the one commit of this PR? I didn't see any test suite that I should add to. If there are other tasks, please just let me know. In any case, this change can be tested by issuing a variety of swaymsg commands, such as:

swaymsg move position left 10 bottom 10
swaymsg move position right 0 top 10
swaymsg -- move position -5 top -5
swaymsg move position right 10ppt 20ppt
swaymsg move position 3px 5px