Fixes & Cleanup

Feature changes:
- Exiting Chkdraft (the whole program, not just an individual map) now
checks for unsaved changes

Bug/Error fixes:
- Fixed an error where the MDI frame would draw (or undraw, rather) part
of the status bar's top frame (looked like part of tree view was there
when all maps were closed, ancient error)
- Got rid of the keybeep when working with tree items (ancient error)
- Fixed an error where toolbar boxes like layer would receive persistent
focus (layover from Classing Up the GUI Code)
- Fixed a bug where cut would not work under most conditions

Code changes:
- Removed all program specific window messages in favor of class
functions (yay ClassWindows!)
- Adapted & moved almost all code from GuiAccel into the WindowsUI
classes