screenshot: make selection area modifiable via keybinds

[nnyyxxxx]

resolves #980

[YaLTeR]

Thanks.

Your original implementation had some problems:

• moving outside the boundaries would reset the selection to the default rather than clamping
• resizing ignored the change and instead used a hardcoded value
• moving and resizing didn't take output scale into account

So I reimplemented it to do all of this.

Now, having tried it, the next logical steps are:

• support switch-preset-window-width/height
  • basically do the same as layout/floating.rs. Will be a bit annoying to keep track of preset widths/heights so that they're the same as in the layout Options
• support move-window-to-monitor-right/left/up/down to move the selection across monitors