webadmin: resize columns

- Made resizing columns in the grid easier by increasing
  the size of the area that can be grabbed from 3 to 5 pixels.
  As well as making the area available left and right of the
  splitting line.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1037477

Change-Id: Id3d89413e5d3376b2c8ac29ecfbac58e2eead68f
Signed-off-by: Alexander Wels <awels@redhat.com>