fix multi-column pane resizing issues

- On setting the number of columns for a single pane, trigger processResize
to ensure that the displayed width of the pane does not exceed the number
of alotted columns nor the space available
- On add or remove column, adjust multi-column panes so that they fit within
the alotted columns and in the space available
- On resize window, adjust multi-column panes so that they fit within the
alotted columns (in addition to fitting within the space available)