KMessageBox: Use question mark icon for question dialogs

Summary:
Besides Information, Warning and Critical, the QMessageBox::Icon
enum supports icons of type Question. Instead of falling back to an
"information" icon, use a "question mark" icon for every
KMessageBox::question*().

Note: Using the Oxygen icon set still displays an "information" icon
(may need investigation).

Test Plan:

• kmessageboxtest now shows question mark icon for some dialogs
• cursory look at lxr suggest most usage is a proper question indeed
• question mark icon supported in Breeze and Adwaita icon set

Reviewers: Frameworks, cfeck

Reviewed By: cfeck

Subscribers: alexeymin

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7859