Fixed tab focus ordering in NodeEditor.

It's still possible to use a ListContainer in such a way that you get bad focus orders, but it's fixed enough for the purposes of the NodeEditor, which is the only place bad ordering has been reported to date. See comments in ListContainer.py for possible alternative approaches that may hold some promise, but were fruitless so far.

For the most part, fixes #107.
Improved GadgetWidget focus behaviour.

It no longer accepts focus from tab events, as that's confusing. It steals focus and releases it on mouse enter and leave events, as requested in #119. I experimented with painting a border around the GadgetWidget (viewer and graph editor) when they had the focus, but I wasn't particularly keen on the way it looked. Will await feedback before going any further with that.
Improved appearance of focussed buttons.

They now have a highlighted border whereas before they looked the same as any other button.
Merge pull request #410 from johnhaddon/tabFocusOrder

Tab focus order