Remove background_color_ in Insets: it takes a lot of unnecessary memory,

Remove background_color_ in Insets: it takes a lot of unnecessary memory,
especially in math where each equation has many insets.

	* src/Insets.cpp: remove background_color_ member
	(setBackgroundColor): remove
	(backgroundColor): make virtual

	* insets/InsetBranch.cpp:
	* insets/InsetNote.cpp: move color setting code from setButtonLabel to
	backgroundColor.

	* insets/InsetBox.cpp (setButtonLabel): remove call to setBackgroundColor.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18750 a592a061-630c-0410-9148-cb99ea01b6c8