* Rename some members/functions to clarify their meaning.

* Ensure that the position for the proposal widget is at start of the
  identifer, so that the filter prefix will be found correctly in the
  GenericProposalWidget. For certain cases the completion were
  calculated but the widget was never shown:

  Case 1:

    void f()
    {
        <COMPLETION_CURSOR>
    }

  Case 2:

    void f()
    {
        st<COMPLETION_CURSOR>
    }

  Case 3:

    if (true)
      <COMPLETION_CURSOR>

  Case 4:

    foo. mem<COMPLETION_CURSOR>

Change-Id: Ie79e01e8a22f8ec306136ec4ccbfffd544edd573
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>