This implements ApplySetting for the proper background of the
undocked SfxDockingWindow.

In addition, like all other code, this sets the text color via
the EditEngine font objects and therefore reverts the uneeded
EditEngine interface from the original commit ea191cf9
("tdf#90297 use field colors for math's edit engine").

Change-Id: Ib7df4ce730da92a1e41f58b8348a5aec18208166
Reviewed-on: https://gerrit.libreoffice.org/72312
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>