simplify widget registration with bqt manager

[hannesdelbeke]

• expose command to bqt.add
  current one is too verbose

it automatically works if you disabled qt wrapping for bqt by setting the env var BQT_DISABLE_WRAP to 1
it assumes you parented all widgets with bqt.widget_manager.add(my_widget)

• add instructions to README on how to use bqt
• inform users that blender_widget is legacy

[hannesdelbeke]

need to decide on ease of use method name

currently we have both

• bqt.add this is just a wrapper for the next one
• bqt.manager.register

bqt.add is short and sweet, saves time typing and easy to remember.
but do we need it?
decide before we start plastering it all over our docs

[hannesdelbeke]

clash with addon register method