Fix an false positive assertion with context menu

Due to the asynchronous nature of context menu events in chromium,
we need to filter out the Qt generated events, and rely of what comes
back from the chromium side. We keep a flag to ensure we're always
in a consistent state, but the part re-setting it in QWEView went missing.
This simply re-adds it together with initializing it to false upon view
construction.

Change-Id: I5d187faf93742898c96a1951b7dc477ce1b3f5bd
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>