TVB-2745: Fix overriding of tvb folder

Created by: adrian-dordea

This was reported by an external user. He was trying to use TVB in ~/TVB/TVB_Distribution folder, and also keeping the default settings for TVB. This means that folder ~/TVB is overwritten when TVB starts after initialization, leaving the process in a bad shape (tvb running, but python files partially removed). With this PR we want to validate on settings page that a user can not select a non empty folder for storing TVB data