Add mobj_props_auto_apply option to preferences dialog
Properly detect folders dragged to the start page
Bump version to beta 4
Add undo history panel to map editor window

- Shortcut key Ctrl+3 (script editor changed to Ctrl+5 to keep things consistent with the main window)
- Add 'Time' column to undo history panel
Merge branch 'master' of https://github.com/sirjuddington/SLADE into Music
Display IMF song information, fix Adlib sounds