Introduce QWebEngineProfile API

Introduces initial widgets API for the Chromium BrowserContext.

Adds API for controlling cookie jar policy, user-agent string and
cache and persistent data paths.

Similar QML API will follow in another patch.

[ChangeLog][QtWebEngineWidgets][QWebEngineProfile] New API for profiles
applying to groups of QWebEnginePages.

Change-Id: I3c4ef4053fde7564af29178c91a0aca8a2b61a5f
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>