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

When the active workspace is changed, marco figures out which window
should get focus by calling `focus_ancestor_or_top_window`. In some
cases that call might include a window that should not get focus as
`not_this_one`.

When `not_this_one` refers to a window, the function will check to see
if it has a parent, and if it does, it will ignore the new workspace's
`mru_list` and will focus that parent window. However, it doesn't check
to see if the parent window is actually on the new workspace. If the
parent isn't on the new workspace, focusing it will drag it over,
including the transient window that was supposed to be ignored. This
isn't the result a user would likely expect, and is made more confusing
by the parent window being minimized, stuck that way until the user
switches to another workspace and back.

This change makes `focus_ancestor_or_top_window` ignore the
`not_this_one` window if it isn't on the new workspace's `mru_list`.
Instead it will just search the new workspace's `mru_list` for a window
to focus.

In addition to existing properties use also new _GNOME_WM_STRUT_AREA
property that allows creating struts between monitors.

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/922de13

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*$//' {} \;

A prior commit switched from focusing the topmost window as the default
window to focusing the MRU window. This was done in alignment with the
introduction of per-workspace MRU lists to avoid problems where the window
stack was inadvertently changed when focusing windows during window switches.

Now that focusing windows don't have as big an impact on the stacking order,
we can revert back to focusing the top window, which is less confusing to the
user.

For now, leave per-workspace MRU lists, as they're a pretty good approximation
of a global MRU list, and it works well enough.

https://bugzilla.gnome.org/show_bug.cgi?id=620744

Based on commit https://gitlab.gnome.org/GNOME/metacity/-/commit/f628d8f8901f46fa9e00707ae9d7ccfd1e85f427

Use g_list_free_full

Possible keybinding to switch to previous workspace.

Since meta_workspace_invalidate_work_area() frees the edges
workspace->screen_edges and workspace->monitor_edges, we must clean up
our cached edge resistance data when the invalidate_work_area() is
called on the active workspace, or when the workspace changes.

Make the computation of the edge resistance data lazy so that it
will be recomputed the next time we try to access it.
meta_display_compute_resistance_and_snapping_edges() is made
private to edge-resistance.c

Invaliding the data when active workspace changes also will improve
correctness for edge resistance when the current workspace changes
during a grab operation. (Even with this fix we still don't try to
handle window positions changing during a grab operation; that can't
cause a crash since, unlike screen and monitor edges, the window edges
are freshly allocated, it will just cause slight oddness in that
corner case.)

Root cause tracked down due to much effort by Jon Nettleton.
https://bugzilla.gnome.org/show_bug.cgi?id=608800