Group/Tab Rework

[vaxerski]

Reworks how groups work internally, making them independent of layouts, allows for floating them, etc.

Fixes: #1576 #770
Closes: #593 (impossible to achieve, superseded)

Breaking changes:
dwindle:col.group... -> general:col.group...

Behavior changes:

• groups are now always created as singular (they don't "eat" neighbors)

[fufexan]

Found a bug.

1. Group 2 windows while in dwindle.
2. Switch to master.
3. Open a new window. It will be behind the group (but focused) until you move the group, afterwards they get tiled.
4. Play with tile/fload. Now the group sits in an awkward position, not being able to be tiled again.
5. Fullscreen/unfullscreen the group. Now it won't even be visible anymore. (optional)
6. Switch to dwindle again. Group is visible and tiled again.

Basically it's borked on master, but works fine on dwindle.

[vaxerski]

was a hard ASan fault for me, fixed now and can't repro your bug. Check plox.

[fufexan]

Nailed it boss.

[vaxerski]

cheers