Adds GNOME3-style dynamic workspace management. Workspaces are created
when no empty ones exist and removed when multiple empty workspaces are
present, maintaining exactly one empty workspace at the end.

Fixes #406

This allows marco to respond to HiDPI scale changes without requiring
a restart. When the scale changes, marco now automatically updates all
icons (windows, xprops, alt+tab, etc.) and mouse cursor to be at the
correct scale factor.

Window decorations are handled through an internal GDK signal triggered
from XSettings changes, and so already work out of the box.

In addition to existing _NET_WORKAREA property set also new
_GTK_WORKAREAS_Dn property where n is desktop number (between 0
and _NET_NUMBER_OF_DESKTOPS - 1).

https://mail.gnome.org/archives/wm-spec-list/2018-December/msg00000.html
https://gitlab.freedesktop.org/xdg/xdg-specs/merge_requests/22

origin commit:
https://gitlab.gnome.org/GNOME/metacity/-/commit/3d8b03d

find . \( -name '*.h' -o -name '*.c' \) -exec sed -i 'N;/^\n$/D;P;D;' {} \;

find . \( -name '*.h' -o -name '*.c' \) -exec sed -i 's/[[:space:]]*$//' {} \;
find . \( -name '*.h' -o -name '*.c' \) -exec sed -i 's/\t*$//' {} \;

Instead of converting from surface to a GdkPixbuf and then back to
a surface, we keep it as a surface for the entire manipulation flow.
This improves rendering speed a bit and sets the ground for a higher
resolution thumbnail in the future.