Add default spacing for Layouts (#5614)

* introduce B_USE_DEFAULT_SPACING, which works somewhat like B_SIZE_UNSET and B_ALIGN_HORIZONTAL_UNSET
* introduce static float BControlLook::ComposeItemSpacing(float spacing), which checks uses be_control_look->DefaultItemSpacing().
* modify layouts to use BControlLook::ComposeItemSpacing() in SetInsets and SetSpacing methods.
* default insets are still 0, 0, 0, 0, but can be set to default spacing by passing B_USE_DEFAULT_SPACING
* I've found two regressions, patches incoming, please report others on #5614.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@38512 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/os/interface/SplitLayoutBuilder.h

@@ -11,7 +11,7 @@ class BSplitLayoutBuilder {
 public:
 								BSplitLayoutBuilder(
 									enum orientation orientation = B_HORIZONTAL,
-									float spacing = 0.0f);
+									float spacing = B_USE_DEFAULT_SPACING);
 								BSplitLayoutBuilder(BSplitView* view);
 
 			BSplitView*			SplitView() const;