1.2-stable+3.4.1

This is a feature-packed release of Goost.

Notable features include:

• A MidiPlayer to play MIDI files with SoundFont: #157.
• A Debug2D singleton to help the process of debugging in 2D by drawing: #162.
• A Spawner2D node to automatically instantiate nodes from scenes and scripts in 2D: #165.
• A GoostMath singleton: #153, #155.
• A PolyEllipse2D node, which generates ellipse polygon, and corresponding GoostGeometry2D.ellipse() method: #167.
• A project setting for configuring path to repository by Git plugin: version_control/git/repository_path.
• An editor setting for MixinScript auto-switching behavior: text_editor/files/open_first_script_on_editing_mixin_script (disabled by default now).