Bug 1005471 - Scratchpad "Jump to line" should preset input value based on current selection, handle LINE:COLUMN as well

Address https://bugzilla.mozilla.org/show_bug.cgi?id=1005471#c9

I hope it's OK to discuss this bug as part of a pull request.

I will work on the test case next.

Let me know if you still see issues with this particular commit.