Message column not resizable

[dhood]

The change in ros-visualization/rqt_common_plugins#398 made it so the message column is not resizable by the user (Stretch mode documentation says "The size cannot be changed by the user or programmatically.").

This is a problem when rqt starts as a small window, e.g. on small screens. Here's a screenshot from a 1366 x 768 monitor:

As you could imagine, I was annoyed that I couldn't resize the small message column (and surprised that it behaved differently to the other columns).

Perhaps it's sufficient to just change the default column width to something larger than the other columns?

[dirk-thomas]

Please consider making a pull request to satisfy both use cases:

• make the initial size of the column big enough to use the available screen space
• allow for manual adjustment by the user afterwards