SegIDHelper interface does not work for everyone

[pllim]

This is a follow up of #47 .

• Font size is hardcoded, resulting in misalignment on my machine (screenshot below). To fix this, adding an interface for me to set font size would be desirable. Or whatever setting that needs to be customizable to make it work, not necessarily font size alone.
• While we're adding configuration, option to change text colors would be nice too, in case you need to display the image in a different colormap.
• Not sure why the dialog for this plugin is on the left, and not right like everything else. Can we move it to the right?
• When I close the plugin, I cannot make the text overlay disappear.
• How do I hide the text while plugin is still active? This overlay can be confusing if I decide to load another image that is not the "segment ID" format.
• Could use module docstring and a Help button to display usage info for users.

[mperrin]

Not time urgent; I will deal with this in June or later, once I'm not on research leave.

[pllim]

Turns out I forgot to clear quipcache, so I was overlaying on older mosaics. When I regenerate them, the misalignment is still present but no as bad as before. I updated the screenshot above. FYI.