Generate the GTK button stylesheet from button_style

Asked for after the previous entry declined to write GTK CSS. The objection
was to clobbering a file full of hand-written work, not to the feature, and
splitting ownership solves both.

srdwm owns srdwm-buttons.css, generated from
theme.decorations.title_bar.button_style and rewritten on every start and
config reload, and adds exactly one @import line to the user's gtk.css if it
is missing. Nothing else in that file is ever touched. The import goes first
because CSS only permits @import ahead of other rules, which also leaves the
user's own rules last and therefore able to override the generated style.

The generated CSS sets a background per button rather than un-hiding a child
image, for the reason the earlier hand-written attempt found by screenshot:
WhiteSur paints the control as the button's own background-image from a
compiled gresource, so clearing that background leaves a blank button rather
than revealing a glyph.

Verified end to end on a real GTK app, both directions, through the
generated file: traffic_lights gives Nemo coloured dots, traditional gives a
dash, a square and an X, each matching srdwm's own titlebar directly above
it. Also verified the import is added once and not duplicated, that
switching style rewrites only the generated file, and that a home without
GTK config directories has nothing written to it.

527 tests pass, clippy clean.

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