Tab Ceiling per brp

This is a very simple add-on I made to control my tab addiction. It does four things:

1. Change links so that they won't open in new tabs on simple sites. This can be turned off in settings.
2. Close new tabs when they open above the user-specified "tab ceiling".
3. Do not close new tabs opening after a session restore.
4. Provide a settings pop-up to allow configuration of link changing and tab ceiling.

Usage:
1. Install the add-on
2. Open the add-on widget from the toolbar
3. Configure number of tabs to limit to, and whether links should be overridden.
I personally prefer the config that links are not overridden, because I'm afraid of losing state when the override unexpectedly opens a link in the current tab.

On FF Android, addons are unable to access the current number of open tabs, and the tab count reflects the number of activated tabs. Tabs will not be blocked on reactivation. If they are, please file a bug.

This add-on is HIGHLY experimental on Android. It seems to behave correctly on FF desktop, and I'm still trying to get it to work on FF mobile. That said, if you want to test it and find an issue, please file the issue on github.