Set Qt::StrongFocus on Quick preview widget.

This is supposed to set the correct focus policy, but in fact due to
Qt-Bug 34414 this currently just affects tab-focus.
But at least it makes focus work correctly as soon as the bug is fixed
in Qt.
Add possibility to call Q_INVOKABLES.

The methodContextMenu() method used to have a check if the method is a
slot before providing the option to invoke. This commit extends this
check by also providing invoke for Q_INVOKABLEs.

Moreover the method now returns if there is no action to take instead
of trying to open an empty context menu (which resulted in an assert).