Godot default addons

Default set of addons that can be added to newly created project (feature can be found on my fork of Godot).

Because these addons are copied with empty/clean project, there are no .import files created for resources. We need to import them manually using function Editor.import().

• arrow_shape: cleanup file naming
• arrow_shape: fixed and modifed for Godot3; Editor.import() added.
• debug_overlay improvments and cleanup
• metadata_inspector
• plugin_refresher
• polygon_builder
• perf_monitor
• sprite_quick_offset
• vector_icons

Removed or replaced by native implementations:

• add_primitives: fixed and updated for Godot3 (editor integration)
• godobuf: fixed and modifed for Godot3
• platform2d
• rmsmartshape2d
• scene_scatter paint with whole scenes; Editor.import() added.
• scrolling_background