editor: altered the button hover style
PackedEditor: render() now returns the rendered buffer
editor: fixed "synth" block_identifier numbers
SynthSampleGenerator: added after_block_callback and setAfterBlockCallback()
editor: addec main_canvas object and basic canvas handing
editor: added callback_synth_sample_generator_block(), render_samples() and actual sample rendering using SynthSampleGenerator, added dependencies in editor.html