Implement Window Functionality

[mwerezak]

Windows

• [x] add_window
• [x] get_windows
• [x] get_active_window
• [x] set_resize_callback
• [x] set_primary_window

Window Menus

• [x] add_menu_bar

Main Window

• [x] set_main_window_pos
• [x] set_main_window_resizable
• [x] set_main_window_size
• [x] get_main_window_size
• [x] set_main_window_title
• [x] enable_docking

[mwerezak]

Moved add_menu and add_menu_item to #5 since they aren't restricted to windows and can actually contain other things

[mwerezak]

Complete!