Corner-tiled windows were being treated as maximized horizontally, which
is wrong. Instead, we clear all maximized flags to make sure their size
is respected on reload.

Fixes #710

This prevents race conditions with applications like mate-screenshot
that need to grab input (e.g. for area selection).

Fixes mate-desktop/mate-utils#37

Replace putenv-based child setup with glib environment functions. This
enables posix_spawn for keybindings and it fixes an old memory leak with
not being able to free the string we added to putenv.

There was a 10px difference in the calculation between the (correct)
center-new-windows placement and the (incorrect) move-to-center
keybinding. By using the same function in both places we can make sure
the calculation is consistent.

Extend MetaFrameBorders to include shadow and resize borders to
calculate invisible border size. Also added border caching to prevent
recalculating borders on every frame operation.

This is a backport of the following commits from Metacity:
- https://gitlab.gnome.org/GNOME/metacity/-/commit/21e664af
- https://gitlab.gnome.org/GNOME/metacity/-/commit/3c6e269b
- https://gitlab.gnome.org/GNOME/metacity/-/commit/700e87ea

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 systems with bamfdaemon running, windows get this property for
finding the dekstop spec file of an application. This can be used to
find the defined icon for any application window that reports this
attribute and provider much better icon matching.

Looking up GDesktopAppInfo from the GTK Application ID we can get a much
better match for the icon and load it at the appropriate scale. This
results in matching icons to straneous windows and better looking icons
overall.

GTK+ has support for the application ID property. This will allow Marco
to reconstruct the path to the desktop spec file and so have access to
all internal attributes (e.g. name, icon, etc.)

Based off Metacity 's commit 7c1cc3ca1d8131499b9cf2ef50b295602ffd6112 [1].

[1] https://gitlab.gnome.org/GNOME/metacity/-/commit/7c1cc3ca1d8131499b9cf2ef50b295602ffd6112