Gradle ports

- Ported Android and Core projects to Gradle
- Ported Desktop project to Gradle
Solved some system dependency issues
Directory changes, generate trees for all tags
Major refactoring/documentation
Added scene graph object stubs

- Added object stubs for scene graph implementation
- Added scene light object stubs
Implemented numerous tree functionalities

- Added various tree capabilities
- Added search functionality to the tree
- Add node function implemented to get complete binary tree
- Tree depth getter
- Separated TreeNode into its own file
- Changed package of Tree objects
- Implemented tree iterators
- Added remove node functionality to tree
Got all libgdx objects inside Scene
New object PhysicalEntity: it has a pose but not a model
Added readme
Adapted camera structure to scene graph
Ignore libgdx.so
Physical camera transform refactoring
Render refactoring
Desktop webcam interface bug fixes

- Fixed #2
- Fixed #3
Added UI elements for mode selection
Made the whole scene graph serializable
Added YAML serializable landmark configuration
Tree refactoring

- Correction of terminology in tree
- Added depth calculation to tree
- Clarification in tree iterator name