Allow making the panel completely transparent if the background
provides enough contrast for either black or white text.

The following background characteristics are used to determine the
text-friendliness of the wallpaper:

 - Normal: The background in the area around the panel does not have any
notable properties (default case, no particular CSS class)

 - Bright / Dark: The panel background is either notably bright or dark
("background-bright" or "background-dark" CSS class)

 - Noisy: The panel background is noisy and might not provide sufficient
contrast for text, this case is also used if the color analysis failed
("background-noisy" CSS class)

A background image can have both the Noisy and the Bright / Dark
characteristics set at the same time.

We use the opaque panel by default and only switch to the transparent
panel if the background is either bright or dark. If the background is
noisy, the opaque panel is always used. In the overview, we always use
the transparent panel with white text, except when there's no window
near the panel and the background is neither bright or dark, to avoid
the unnatural transition of the background getting darker while the
panel is getting brighter.

The "window-created" signal of MetaDisplay is used instead of connecting
to the "actor-added/removed" signals of the window_group because the
workspace switching animation reparents all windows to its own container
during the animation, which causes a lot of unnecessary calls to
_updateOpaqueState.

Fixes #408