Re-added LineEdit interface. Honestly I'm not too thrilled with how the unparent/reparent system is turning out; removing it failed, however...
Added the framework for tabs, added necessary assistant routines for tabs, and implemented those assistant routines on the Windows backend.
Implemented Tab on Windows.
More TODOs.
Removed extern from C function definitions in containers_windows.c.
Simplified the control nesting model by removing Control.unparent() and requiring all Windows to have a Control at construct time; implemented such on Windows.
Adjusted sizing data to act as the base container type; this is needed for proper resizing on both GTK+ and Mac OS X.
Updated the GTK+ backend to have the new parenting and sizing changes. Now to implement Tab! ...though I should probably implement the changes on Mac OS X first.
Implemented the various parenting and sizing changes on the Mac OS X backend; also moved a line in window_windows.go around for consistency.