interface freeze when accessing a slow network share

When expanding a network share in the left-hand pane, Mixxx tries to enumerate the directory content.
When it is a network share on a slow connection, or if it is an unaccessible network share, the interface freezes.
When the interface freezes, playback continues without problem, but you can't have any interaction with the program.

Steps to reproduce:
1. mount a network share on your system (either NFS for Mac/Linux or assign drive letter under windows). the problem is mostly seen on a network share with tons of subfolders.
2. add some sort of bandwith management to simulate slow access to network share
3. launch Mixxx and expand network share mountpoint in the left-hand pane

Actually seen under windows seven and windows 10.

Actual result: interface hangs waiting the treeItem to fill with all content

Expected result: no interface hang and a visual feeback to user that content is being collected in the background