Cleaned up a bunch of TODOs, moving a few to future plans.
Removed GTK+ spurious resize TODO and debug spiel; I can't notice much of a difference after ignoring extraneous resizes. The custom container will stay, however, as it does not demonstrate weird background drawing bugs with oxygen-gtk.
Wrote the documentation for Go(). Also more TODOs.
Decided to consider the preferred size of all tabs on Windows; on Windows Tabs aren't really expected to resize based on which tab is selected, unlike Mac OS X...
Merged xsizing_darwin.m into control_darwin.m. Even though alignment info is only currently used by Label, these functions are called in basegetAuxResizeInfo(), and who knows what will happen future.
Added C99 to the Windows and GTK+ cgo CFLAGS after Ian Taylor confirmed it was safe to use.
Stripped C-style comments from C sources and headers where appropriate.
Fixed Tabs on Windows having undesirable tab stop behavior. This doesn't work correctly in wine, and I can confirm for the first time that it actually is a bug in wine!
Fixed the infinite loop on Tab control dialog message bug(s) on the Windows backend.
Moved TODO about Labels to mergeback/grid.go; will worry when Grids become a thing again.
Fine-tuned margin/spacing values on Mac OS X. Also more future plans.