Add structure for tags and displays

Add structure for tags and displays

These don't actually do anything yet..

Each tag has multiple displays, but all the tags
should have the same number of displays with the
same root windows.

tag 0:
  +-----------+
  | display 0 |
  |           | +-------------------+
  |           | | display 1 | col 2 |
  |           | |       |   | win 0 |
  |           | | col 0 |   |-------|
  |           | |       |   | win 1 |
  +-----------+ +-------------------+

wm: {
	tag 0: {
		display 0: {
			col 0: {
				win 0:
				win 1:
				win n:
			}
			col 1: ...
			col n: ...
		}
		display 1: ...
		display n: ...
	}
	tag 1: ...
	tag n: ...
}