Import QtWebKit public headers.

This is the first step toward re-implementing part of the QWebView
API on top of QtWebEngine. The plan is to import the complete headers
to facilitate diffs and progress tracking.

Changes squashed in this commit:
- Use the QWebEngine prefix for class names
- Strip out non-public members and directives
- Allow building using those headers by disabling the Q_PROPERTY
  macros and by adding a dummy implementation for virtual methods
  directly in the header
- Update the widgetsnanobrowser example to comply with the slight
  changes from the previous API

Change-Id: Ia7efa5430f775d09b493544430a04856cc7928f6
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>