Decoration can announce whether it currently requires an alpha channel

A decoration can provide the AbilityAnnounceAlphaChannel in addition to
AbilityUsesAlphaChannel. If this ability is provided the decoration can
enable/disable the use of the alpha channel through setAlphaEnabled().

The base idea behind this mechanism is to be able to tell the compositor
that currently alpha is not needed. An example is the maximized state in
which the decoration is fully opaque so that there is no need to use the
translucency code path which would render all windows behind the deco.

In addition also the blur effect honors this setting so that behind a
known opaque decoration no blurring is performed.

Oxygen is adjusted to disable translucency in maximized state and Aurorae
is adjusted to allow themes to enable/disable translucency. For Plastik
translucency and with that also blurring is disabled.

REVIEW: 106810