This commit adds functionality that will try to open new tabs in the same container as the previously active tab in that window, if the new tab doesn't already have a container, and the previously active tab did.
Some undocumented / implicity functionality in the webextension API is used:
* `onCreated` is called for a new tab before `onActivated` for that tab is called
* `onUpdated` will be called for all new tabs, also `about:newtab` and the like (sets the favicon)
* The first `onUpdated` for a new tab, that is not an `about:`-tab, will set the URL
It is a little bit wonky, as the original new tab is created and displayed (although not loaded) before it can be closed a new tab can be created. However, it is the best workaround I can find to add this functionality until Bugzilla 1406371 is solved.
This would fix#462, #448 and #406 (I think)
Also relates to #943 and #544
