Don't stop MIDI playback in AudioEntryPanel dtor

This is not consistent with other types of audio playback, and didn't
care whether the MIDI being played was actually from the destroyed
panel.
Add ability for NumberTextCtrl to accept decimal numbers
Make OGLCanvas refresh timer settable
Fixes/improvements to LinePropsPanel

- Save sector tag correctly
- Implement 'New Tag' button
- Add line id field (for UDMF)
- Use NumberTextCtrl for number text boxes
Add custom properties dialog for things
Merge branch 'master' of https://github.com/sirjuddington/SLADE.git
Merge branch 'master' of https://github.com/sirjuddington/SLADE into Music
Update VC++ 2005 project file.
Removed some unused code.
Allow prev/next track buttons to wrap around.