* **Two-axis tiling resize**.  You can drag a 24-pixel window corner (purple) to perform a two-axis resize. Also, you can press Mod and drag a large corner inside the window (one-third of the window size).
  ![illustration](https://user-images.githubusercontent.com/5121426/75146543-40a7c580-56f3-11ea-9b26-e14718699323.png)

* **Live resize**: the tree is rendered during the resize process.

* You can still press **ESC to cancel** resizing and revert to the old state.

* Introduced the concept of the **minimum container size**.  E.g. the minimum width of leaf con with 10px borders is 21px (left border + right border + at least 1px for the window itself).  The minimum size is defined recursively, e.g. the minimum height of the vsplit is the sum of minimum sizes of its children.  The reason for this change: the live resize made it very easy to shrink containers as small as possible, so some restraints are required.

* ~Misc update: ignore the drag if i3 can't perform the resize.  Suppose the user tries to resize the rightmost window to the right using Mod+RMB drag.  Obliviously, i3 can't do this.  Before this PR, the drag has been propagated to the application (some apps would show a context menu).  After this PR, the drag would be ignored at all (and the cursor would be shown as ✕).  The reasoning behind this change: the user explicitly indicated their desire to resize the window by pressing the Mod key, and probably just miss clicked to the wrong part of the window.~ Done in [93e96f4](i3@93e96f4).

* Bugfix in `tiling_resize()`: you can now resize the window with pixel border by dragging the top border.

* enum border_t -> enum resize_direction_t
* resize_cursor() -> xcursor_type_for_resize_direction()
* resize_get_borders_sides(), resize_get_borders_mod() -> get_resize_direction()
* resize_find_tiling_participants_two_axes()

Same fix as in this commit from 2020 that was previously mentioned in 
the PR discussion.
i3@93e96f4