FIX: Solves most of compilation errors on clang

CMakeLists.txt

inc/lektor/common.h

inc/lektor/lib/strv.h

src/module/module_qt_window.cc

src/module/module_qt_window.hh

src/module/qt_window/mainwindow.cc

src/module/qt_window/mainwindow.hh

src/module/qt_window/mpvwidget.cc

src/module/qt_window/mpvwidget.hh

src/module/qt_window/mpvwidget_interface.cc

src/module/qt_window/mpvwidget_interface.hh

src/module/qt_window/qthelper.hh