This changes the way zero (which is the default) is interpreted for both
the width and height of `floating_maximum_size`. It now refers to the
width and height of the entire output layout, which matches i3's
behavior.
This also removes duplicated code to calculate the floating constraints
in three files. Before this, `container_init_floating` used two-thirds
of the workspace width/height as the max and the entire workspace
width/height was used everywhere else. Now, all callers use a single
function `floating_calculate_constraints`.
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