Tab-related changes

I fixed and added shortcuts to navigate the tab bar; also the tab bar ordering was reloading in reverse-order and I made the previous session's tabs append to the end to fix that (e.g. before tabs A, B, and C would load C, B, A, then quit as C, B, A, then load as A, B, C, ad infinitum).  Another way to fix it is to loop in reverse in Init(), but that clashes with FileUsed()/RememberOpenFiles() in LoadDb().

Tab order is still wonky if you drag-and-drop them, but supposedly this is fixed in 2.9.5 (see http://trac.wxwidgets.org/ticket/10848); it's not yet in my repos for me to test though.