InterfaceDefs: Adjust and introduce new spacing/insets constants.

* Nothing in the tree and few things outside it used BIG_{SPACING|INSETS};
   it seems a value of 15px (at default font size) is not that useful.
   There are, however, a lot of things around the tree that use multiples
   of 20px. So, make BIG be that, with the intent to replace those
   with BIG directly.

 * Introduce CORNER_{SPACING|INSETS}. There are a lot of applications
   (e.g. Tracker, Terminal, Debugger etc.) which use scroll bar width/height
   to metrically align controls with the window frame or with some other
   control which contains scroll bars. Rather than have to invoke
   BScrollBar or BControlLook directly to get the value, we should just
   derive the size of scrollbars from a spacing constant instead
   and get rid of the custom function. (For now it is just replaced.)
   This reuses the old values for BIG, as it is equal to 14px at default.

 * Introduce BORDER_{SPACING|INSETS}. This is equal to the typical border
   size of 1px at default font size (or lower) and uses floor() instead of
   ciel() to compute what the size should be (i.e. it will remain 1px
   at 150%/18pt and only go up at 200%/24pt.) This will allow a lot of
   the hardcoded border sizes around the tree and elsewhere to use
   ComposeSpacing() instead.

Change-Id: Iaea3fa30364859888e816a9d61ac156268d70758
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5702
Reviewed-by: waddlesplash <[email protected]>
Reviewed-by: nephele <[email protected]>
Tested-by: Commit checker robot <[email protected]>

headers/private/interface/HaikuControlLook.h

@@ -337,8 +337,6 @@ public:
 									uint32 flags = 0,
 									uint32 borders = B_ALL_BORDERS,
 									orientation orientation = B_HORIZONTAL);
-	virtual float				GetScrollBarWidth(
-									orientation orientation = B_VERTICAL);
 
 protected:
 			void				_DrawButtonFrame(BView* view, BRect& rect,