Clean NO_PRIVATE_API usage

Contains two commits. First commit cleans and fixes errors from startup sequence. Second one cleans / removes NO_PRIVATE_API ifndef blocks and related sources.

The first commit contains following changes:
- If mozcontext ready, view created on component completed.
- Creation of render object connected to beforeSynchronizing, no
  need anymore for scenegraph initialized connection. The beforeSynchronizing
  is also good in sense that the GUI thread is locked.
- Due to above GL context used for rendering is bound and QSGThreadObject
  could be made self-contained.

See also https://github.com/tmeshkova/qtmozembed/pull/78