i3-like manual tiling

[ibabushkin]

This feature would be very nice to have. The initial idea is to implement it as a Layout instance. However, as it is now, that's impractical (since Layouts don't carry mutable state).
Thus, the following changes would have to be made:

• Each OrderEntry is a tree of windows and splits, as i3 handles all of it's windows
  • verify that no illegal structure of the tree is possible.
• A Layout is a strategy allowing to do multiple things, namely:
  • Restructure an arbitrary tree into a layout-specific, canonical shape.
  • Handle inserting and deleting windows.
• A "manual" layout would emulate i3's behaviour by inserting windows at the cursor.
• The tree converions mentioned above should follow a convention (ie. the master is always the left-most leaf in the tree, that is, top left).
• So, superficially, everything remains roughly the same if you use the simple layouts already present (like the various *Stack).
• More complicated layouts, like Grid and Monocle would need some additional thought, but a simple convention (or two), combined with the idea that layouts still build the geometries associated with a tree of clients, should make them entirely possible.

Based on the points above, some other aspects of the already existing infrastructure will see some change: Depending on whether the tree holds information on the splits (and I guess it should), focus and window swap can be moved out of layouts, as well as the ugly hack with new_client_as_master (maybe). Client set updates should also happen as soon as possible, to avoid complication.