Changed the way window size/position saving works

Now uses a new 'window_info' section in the config rather than cvars, which will allow the feature to be extended to other windows/dialogs without needing to add a bunch of extra cvars.
Remember the size of various dialogs between sessions

Added new 'SDialog' base class similar to STopWindow, but for dialogs, which automatically uses saved window size info
Merge branch 'master' of https://github.com/sirjuddington/SLADE.git