Feature/cmake cleanup (#381)

* Rename OnScreenGui module to ImGui
* Support multiple external module folders (closes #31)
* Cleaning up CMake files
  * Restructure application specification
  * Add parameter for library mode to all modules
  * Add functions to handle global variable state
* Misc/remove warnings (#383)
* Increase build timeout and do a clean rebuild every commit

modules/globebrowsing/cache/texturecontainer.cpp

@@ -78,10 +78,10 @@ ghoul::opengl::Texture* TextureContainer::getTextureIfFree() {
 
 const openspace::globebrowsing::TileTextureInitData& TextureContainer::tileTextureInitData() const {
     return _initData;
-};
+}
 
 size_t TextureContainer::size() const {
     return _textures.size();
-};
+}
 
 } // namespace openspace::globebrowsing::cache