KFontChooser: add a default constructor that takes a QWidget* first arg
For the sake of the designer plugins, otherwise using KFontChooser in .ui files wouldn't work, see [1].
To make this work, I changed the default values for the args of the deprecated ctor; according to [2] that doesn't break binary compatibility.
[2] https://community.kde.org/Policies/Binary_Compatibility_Issues_With_C++