Implement Basic Widgets

[mwerezak]

Content

• [x] add_text
• [x] add_label_text
• [x] add_separator
• [x] add_progress_bar

Buttons

• [x] add_button
• [x] add_checkbox
• [x] add_selectable
• [x] add_radio_button
• [x] add_color_button - not really a button but a drag and drop control? w/e

Selection

• [x] add_combo
• [x] add_listbox

[mwerezak]

Will implement add_image() with #16

[mwerezak]

Complete!