Resizing of columns when two or more are expanded

["Nickolay V. Shmyrev" <nshmyrev yandex ru>]

Hello all.

Currently resizing of expandable columns is a bit confusing. When you
start drag,
column jumps, increase or decrease it's size in random manner. The
reason is that
when you start resizing, expandable column get width from allocation
that is it's 
new width = visible_width = width + extra space.

Bugzilla entry http://bugzilla.gnome.org/show_bug.cgi?id=169152 has
testcase and patch that makes columns resizing more slick. It calculates
new column size so that new_size + extra_space = visible size. Please
review it.

Probably there is need to implement a bit different behaviour of columns
resizing. Examples from other toolkits: QT hardly restricts such cases -
it only allows mode when all columns are expandable or none of them in
QListView. In QTable expandable column cannot be resized. Swing have 4
modes of resizing, but any of them suggests every column is expandable.
The more natural is resize only rightmost columns and there is also mode
to expand all columns.