Add basic version/update check

- Option to check for updates on startup
- Option to include beta versions in update check
- Menu item 'Help -> Check for Updates...'
Fix VersionCheck when no connection present
Fix potential crash in MapLine::needsTexture
Remove sides with invalid sector references on map load
Add map statistics to map preview panel
Add map size to stats line on map preview panel
Fix wrong 2d mode flat texture appearing after deleting a sector
Merge branch 'master'