Use separate profiles for regular and private tabs

This MR introduces separate profiles for private and regular tabs. Without the separation and dedicated profiles, we had cases when some cookies were leaked from regular to the private tabs.

As a side effect, it adds profile string setting that can be used in future for switching between user profiles. Currently, it is separating cookies and other info stored by QtWebEngine. If developed further, we could later also keep history and bookmarks as per profile.

I am going to look into better handling of httpUserAgent and WebView interaction. It is a bit hit and a miss right now. Probably was before as well. Unfortunately, the user agent is a part of profile, which is shared between tabs with potentially different isMobile settings.

Fixes #33 (closed)