tiles: Adjust padding between windows

Merged Ismael Asensio requested to merge work/iasensio/tiling_half_margins into master

Make the total padding between windows equal to the padding against the screen borders by applying just half of it on the common borders.

I haven't test it on multiple screens but it should also be fine there.

tiling_before tiling_after

BUG: 469720 FIXED-IN: 5.27.6

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