Forward QComboBox signals instead of QComboBox lineedit signals

Summary:
The QComboBox may not be editable when we connect the signals.
When we change the editable flag later on, the signals wouldn't be
connected. Instead, forward the signals available in QComboBox to
mitigate this issue.

Reviewers: apol, dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7968