Add "handles" for resizing columns

[runboxdave]

Resizing columns can be very fiddly, and especially so on mobile devices where it isn't easy to pinpoint the exact spot you need to place your finger and often resulting in no movement of the column boundary or else highlighting text or causing other unwanted interface actions.

It would be useful to add permanent "handles' in a convenient place so that the area that needs to be touched is bigger.

[gtandersen]

@davidbowdley You can also resize columns by dragging the column headers, which are visible at all times.