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/GroupLayoutBuilder.h

@@ -13,7 +13,7 @@ class BGroupLayoutBuilder {
 public:
 								BGroupLayoutBuilder(
 									enum orientation orientation = B_HORIZONTAL,
-									float spacing = 0.0f);
+									float spacing = B_USE_DEFAULT_SPACING);
 								BGroupLayoutBuilder(BGroupLayout* layout);
 								BGroupLayoutBuilder(BGroupView* view);
 
@@ -29,7 +29,8 @@ public:
 			BGroupLayoutBuilder& Add(BLayoutItem* item, float weight);
 
 			BGroupLayoutBuilder& AddGroup(enum orientation orientation,
-									float spacing = 0.0f, float weight = 1.0f);
+									float spacing = B_USE_DEFAULT_SPACING,
+									float weight = 1.0f);
 			BGroupLayoutBuilder& End();
 
 			BGroupLayoutBuilder& AddGlue(float weight = 1.0f);