Settings window slow to open

Extracted from #32

It takes a few seconds for the Settings window to display when opening it. The program is unresponsive until it does.

@mfiano :
I think I might have found why this happen.
When the settings windows opens up it populate fields and one of them is the size of your shares.
So it's doing a directory scan to sum up all the files inside them.
Could you try this branch where I've put this specific scan in a separate thread to avoid the UI to lock up and report if it's working for you ?

git clone -b fix-settingswindow-slowness https://github.com/gfarmerfr/nicotine-plus.git