Fix uncalled callbacks with findText

Calling findText successively might prevent the previous pending
FindReply to be sent, which would leak the callback on the
application side.

This would cause a crash in qupzilla since we empty all pending
callbacks in the QWebEnginePage's destructor to catch this kind
of issue.

This also renames lastRequestId to nextRequestId to make it clear
that this is the ID generator for everything, including findText,
and that lastFindRequestId is only a tracker.

Change-Id: Ia78d553a58ed31af7237aad8772fa9828560c6d4
Reviewed-by: Andras Becsi <andras.becsi@digia.com>