This patch implements the preference "browser.tabs.insertAfterCurrent" which,
when set to true, will cause all tabs (related and unrelated) to be opened next
to the current tab.
It also implements the browserSettings API "newTabPosition", which allows
extensions to control both "browser.tabs.insertRelatedAfterCurrent", and
"browser.tabs.insertAfterCurrent" via values for "afterCurrent",
"relatedAfterCurrent" and "atEnd".
The code for "browser.tabs.insertAfterCurrent" including the test for it is
mostly taken from a patch attached to bug 933532 written by Masayuki Nakano.
MozReview-Commit-ID: KQE7M2FGpc7