Implement Core Engine Functionality

[mwerezak]

Engine

• [x] start_dearpygui
• [x] stop_dearpygui
• [x] is_dearpygui_running
• [x] set_vsync
• [x] get_delta_time
• [x] get_total_time

Callbacks

• [x] set_start_callback
• [x] set_render_callback
• [x] set_exit_callback

[mwerezak]

Custom event loop support is more of a niche use -- split that stuff off into #17

[mwerezak]

Complete!