The layout fill order allows to configure how Cons are attached to a new
layout. The value 'default' keeps the traditional order, filling the
layout from left to right, or top lo bottom depending on horizontal or
vertical orientation. 'reverse' in turn means to fill a layout from
right to left, or bottom to top.

This commit does not yet add the behaviour described above, but merely
extends data structures to accommodate for this new option.

We only store a single fill order per Con and not any additional values
for the workspace layout and last split layout as those variables are
necessary to deal with the default layout which does not apply to the
fill order.

The tree representation was updated to support the new reverse layouts
in the form of 'Vr[]' (N.B. the 'r' suffix). cmp_tree() needs to be
updated to work with those extended strings.

This enables the layout fill order to be saved using i3-save-tree and
restored using attach_layout.

This adds support to specify the default layout fill order for
workspaces. workspace_layout receives an additional optional argument to
set layout fill order to reverse.

Calling 'layout <layout> reverse' allows to change the current layout to
a new layout with reverse fill order.

Allows splitting the current con into all directions by creating new
horizontal or vertical splits with corresponding layout fill order. This
is accomplished by changing tree_split() to accept a direction_t instead
of just orientation_t.

Also convert 543-layout-reverse.t to use subtests to improve structure
and output readability.

With the newly introduced reverse layouts, new windows can appear on the
opposite side of where the split indicator used to be drawn. This
updates the drawing code to draw the indicator closer to new windows.

This was a relatively complicated assignment repeated twice. Factor it
out into its own function.

(see i3#4246 (review)).

reverse argument) and manipulating layout.

method parameters for right-to-left layouts.