Push buttons don't rely on their parent's child transparent mode
since i#38498. This allows buttons to have proper transparency
mode, despite toolboxes don't set their children transparency mode
at all (even being transparent themselves).

However, when other controls are placed to toolboxes, as FixedText,
they check parent's child transparent mode, and then use parent's
background for them. For transparent toolboxes this makes the
background to be NONE, and thus on invalidation of such FixedText,
neither parent's area, nor FixedText's is filled to remove previous
text before outputting new text.

This patch sets ToolBox child transparent mode depending on its own
transparency. Also, FixedText now checks if background applied in
ApplySettings is empty, and if it is, sets transparency mode despite
parent's setting.

Change-Id: Id424cbad894bb42a88a4b027f2a964d6e542f115
Reviewed-on: https://gerrit.libreoffice.org/49621Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>