support per-window Private Browsing

[myrdd]

This will probably also fix #531 and #532.
This issue is part of #526.

from the list on this MDN page RP uses nsIPrivateBrowsingService and observes private-browsing.

Supporting per-window private browsing

Per-window private browsing has been introduced in Fx20, so that will be the minVersion.

[myrdd]

I've tried to implement support for per-window Private Browsing right now, but I've discovered that some bigger changes are necessary for full support.

• Currently, when a user has a browser with the old Private Browsing Mode, at the time leaving the PBM all temporary rules are revoked.
• For per-window Private Browsing it's necessary to have per-window temporary rules – or what do you think?

However, it's easy to forbid adding permanent rules in a window that is private, so I'll go with that option for now.

[myrdd]

I won't implement per-window temporary rules. However, adding permanent rules in a private window is not possible by default. This has been fixed in commit e17c52d already.