GtkTreeView RTL support

[Jonathan Blandford <jrb redhat com>]

Hi,

I've just committed support for RTL GtkTreeViews.  I wouldn't be
surprised if there were bugs in it, but I'm pretty happy with how it's
working right now.  Many thanks to Matthias Clasen for doing the initial
work on this patch.  If those who run in RTL locales could check to
confirm that it's working correctly, I'd be much obliged.

A couple of comments:

 * I've closed #50288.  Please file any other bugs against it
   individually.

 * Part of this patch involved making some changes to the focus drawing
   code.  Again, I'm pretty sure I haven't broken anything but please
   file bugs if they exist.

 * As an added bonus, I added an 'expand' property on
   GtkTreeViewColumns, so that the extra horizontal space in a TreeView
   can go to columns other than the last one.

 * Part of adding the property involved adding an extra bitfield to the
   end of the GtkTreeViewColumn struct.  This pushes the total number
   from 8 to 9.  This does not break ABI compat on any gcc I could find,
   but could potentially cause problems with objects that inherit from
   GtkTreeViewColumn and have eg. a char as the first element.  As the
   rules for bitfields are so ill-defined in the C ABI spec, please let
   me know immediately if this breaks things.

 * This is showing a weird behavior in GtkScrolledWindow.  That is, when
   expanding a row it keeps the alignment of the TreeView in the swindow
   on the left side, meaning the tree-view expands to the right.  This
   feels pretty wrong to me, and (I feel) should be fixed in the
   GtkScrolledWindow.  Will this break other widgets, though?

Comments,
-Jonathan