Improve MainFrame design

[AwesomeTMC]

I think this screen could use improvement. The list is blank when there could be text pointing out what's important: that the directory selected is not valid or you need to select a directory. The galaxy list would show up if the selected directory is valid. Here's a mockup of what it would look like:

[AwesomeTMC]

^ Fixed in the commit above. I thought that would've closed it, but it seems I'm wrong.