Removes the hidden “resized-width” and “use-resized-width” properties
from GtkTreeViewColumn and instead uses the “fixed-width” property to
serve the same purpose.  “fixed-width”, if set, will now override the
auto-sized width (-1 is now a legal value meaning “not set”).

Additional “cleanups” in this commit:

1. When the user resizes the column the “expand” property is now also
set to FALSE, in order to prevent the column from suddenly jumping to a
different width when the window is resized.

2. The code that translated mouse movement to column sizes has been
simplified:
the change in column width is now calculated directly from the distance
the mouse cursor has traveled.  Weird behavior that might have happened
previously if the position of the column changed during resizing, is now
prevented.

3. There was some lengthy logic handling the keyboard shortcuts used to
resize treeview columns, which would call gtk_widget_error_bell() once
the minimum or maximum width was reached.  Instead of rewriting these
checks I simply set the “fixed-width” property to what was requested,
relying on the fact that it is already clamped between the minimum and
maximum width during size allocation.
I will greatly surprised if anyone notices the missing error bell.

https://bugzilla.gnome.org/show_bug.cgi?id=691751