/usr/include/plasma/widgets/ is in kdelibs5-dev 4:4.14.38-0ubuntu3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/plasma/widgets/busywidget.h
/usr/include/plasma/widgets/checkbox.h
/usr/include/plasma/widgets/combobox.h
/usr/include/plasma/widgets/declarativewidget.h
/usr/include/plasma/widgets/flashinglabel.h
/usr/include/plasma/widgets/frame.h
/usr/include/plasma/widgets/groupbox.h
/usr/include/plasma/widgets/iconwidget.h
/usr/include/plasma/widgets/itembackground.h
/usr/include/plasma/widgets/label.h
/usr/include/plasma/widgets/lineedit.h
/usr/include/plasma/widgets/meter.h
/usr/include/plasma/widgets/pushbutton.h
/usr/include/plasma/widgets/radiobutton.h
/usr/include/plasma/widgets/scrollbar.h
/usr/include/plasma/widgets/scrollwidget.h
/usr/include/plasma/widgets/separator.h
/usr/include/plasma/widgets/signalplotter.h
/usr/include/plasma/widgets/slider.h
/usr/include/plasma/widgets/spinbox.h
/usr/include/plasma/widgets/svgwidget.h
/usr/include/plasma/widgets/tabbar.h
/usr/include/plasma/widgets/textbrowser.h
/usr/include/plasma/widgets/textedit.h
/usr/include/plasma/widgets/toolbutton.h
/usr/include/plasma/widgets/treeview.h
/usr/include/plasma/widgets/videowidget.h
/usr/include/plasma/widgets/webview.h