Load a default database with Download DICOM tool

[Zach Pearson]

From an email from Elaine:

(1) Tool starts up with Database: None. Instead of "None" may be more useful to simply put "TCIA" and start loading it in immediately, since there is no other choice yet. Or, if that would make startup too slow, instead of "None" maybe something like "choose database" to make it more obvious.

(2) Even after I choose "TCIA" as the database, absolutely nothing happens. I would have expected it to start loading immediately, and if it takes a while, I would expect a status message like "loading database info..." and maybe (if easy) some kind of constantly updating % progress number.

(3) If I click Search the entries do show up. This doesn't really make sense to me. I think the entries should show up automatically. I thought Search was going to bring up some other dialog to search for a string but it doesn't do anything like that. So if show all the entries is all it does, I'd say nuke that button and just show them automatically. If you absolutely need to have that button, then it should be something like "Load" or "Load Info" instead of "Search"

[Zach Pearson]

In ​this commit I removed the 'None' entry, making the default TCIA. I then added code to automatically load collections and inform the user in the status area that that was happening. The work happens in a QThread so as not to interrupt the UI.