Let Topside remember what files were previously open

[jacobdeery]

Right now, Topside always starts up without anything loaded, and if I want to open a ProcLang file I need to open it from the menu. It would be nice if Topside, when closing, remembered what the last open file was, and then when opening, loaded the same file at startup.

The best way to do this is probably with QSettings - take a look at this documentation for reference (it's C++, but the Python API should be very similar).