Fix tileset saving crash/corruption

Tile.Name is set to null initially, which caused null reference errors and corrupted the tileset upon attempting to save it.  Fixed by explicitly setting Name to an empty string in the Tile constructor.  Also fixed ProjectTree refreshing, somehow the Refresh() call got commented out when I revamped the file watchers and I never re-enabled it.  Here I am, supposed to be fixing bugs and I introduce new ones!