Add window edge snapping setting

[Koranir]

Relies on pop-os/cosmic-comp#1189

Creates a new section in the "Window Managment" page called "Floating Layout", which has the "Window snap-to-edge threshold setting" that defaults to 0 (no edge snapping).

Adds a "Floating windows gravitate to nearby edges" toggle in the "Window Managment" page that sets the edge_snap_threshold cosmic-comp configuration to 10 pixels if enabled, or 0 pixels (off) if disabled.

[Drakulix]

(@pop-os/ux please confirm, if I paraphrase this correctly.)

I think we just want this setting to be a toggle. It should display being "on" on any positive value (we want to allow people to mess with the config file) and "off" for '<= 0', toggling on should set the value to 10 and off to 0.

As a description we want "Display edge gravity for floating windows" instead of "Window snap-to-edge threshold". (@pop-os/ux Maybe the "for floating windows" part is redundant given this is in the "Floating Layout" section?)

[maria-komarova]

I'd rather avoid creating a separate section called Floating Layout since Floating windows is default for any system and people don't really use this term all that often. So let's place the list item under Super key action.

[Koranir]

I've taken the liberty of naming the setting "Floating windows gravitate to nearby edges", which I think represents what it does much better than "Display edge gravity for floating windows" - let me know if I have to change it back.

[Drakulix]

Code wise this looks good!