Code cleanup branch (#618)

* Make height map fallback layer work again
  * Add documentation to joystick button bindings
  * Removed grouped property headers
  * Add new version number constant generated by CMake
  * Make Joystick deadzone work properly
  * Change the startup date on Earth to today
  * Fix key modifier handling
  * Add debugging indices for TreeNodeDebugging
  * Fix script schedule for OsirisRex
  * Do not open Mission schedule automatically
  * Upload default projection texture automatically

  * General code cleanup
  * Fix check_style_guide warnings
  * Remove .clang-format
  * MacOS compile fixes
  * Clang analyzer fixes

include/openspace/util/concurrentqueue.h

@@ -47,6 +47,8 @@ public:
 
     size_t size() const;
 
+    bool empty() const;
+
 private:
     std::queue<T> _queue;
     mutable std::mutex _mutex;