BControlLook: Introduce ComposeIconSize().

This takes an int32 (e.g. B_MINI_ICON or another constant) and
then returns a BSize scaled appropriately, the same as ComposeSpacing()
does already for the *_SPACING constants.

This will be used to replace icon size computations throughout the tree.

headers/os/interface/ControlLook.h

@@ -102,6 +102,7 @@ public:
 	virtual	float				DefaultItemSpacing() const = 0;
 
 	static	float				ComposeSpacing(float spacing);
+	static	BSize				ComposeIconSize(int32 size);
 
 	virtual uint32				Flags(BControl* control) const = 0;