Regression, ProposalWidget is not updated upon pressing left arrow

This commit - 909cb3859b1b68408cbf71354c95b6bf39d15c12, which fixed QTCREATORBUG-15038, also introduced a regression.

How to reproduce the regression:
If you have some text, for example `std::move(something)`:

• put cursor after `::`
• press Ctrl+Space (`ProposalWidget` should appear)
• press left arrow

The content of the window will NOT be adjusted/updated to the first letter, while it should. Essentially `ProposalWidget` seems to be just dead.

My experiments showed that restoring

  if (isVisible())
    d->m_assistant->notifyChange();

in plugins/texteditor/codeassist/genericproposalwidget.cpp fixes the regression and does not re-introduce the bug described here - QTCREATORBUG-15038