General sanity improvements

brot2 has a lot of code debt at the moment owing to the way in which its early versions were hacked together. This blueprint is to repay that debt to provide a saner basis for implementing new features.

In particular, main.cpp is rather unstructured and very Cish C++, due in part to using the C GTK+ interface. To repay this debt, I want to re-engineer it to use gtkmm and to be properly C++ish; have a class for the main window, break the about and preferences dialogs into their own classes (in their own files), and push the menu handling out into its own file(s). Even the existing object structure leaves something to be desired, as the render context maintains copies of items from the Plot2.