Implement tri-state checkbox action in menu

The new TriStateAction class is an improvement of the old one in the
sense that:
1. Have public method to set states.
2. Can connect to the usual Qt slots.
3. Draws checkbox at the correct offset (where QAction draws) in menu
   and better handling of mouse clicking and keyboard navigating.

src/gui/gui.pri

@@ -29,6 +29,7 @@ HEADERS += \
     $$PWD/optionsdialog.h \
     $$PWD/previewlistdelegate.h \
     $$PWD/previewselectdialog.h \
+    $$PWD/private/tristatewidget.h \
     $$PWD/raisedmessagebox.h \
     $$PWD/rss/articlelistwidget.h \
     $$PWD/rss/automatedrssdownloader.h \
@@ -63,6 +64,7 @@ HEADERS += \
     $$PWD/transferlistmodel.h \
     $$PWD/transferlistsortmodel.h \
     $$PWD/transferlistwidget.h \
+    $$PWD/tristateaction.h \
     $$PWD/uithememanager.h \
     $$PWD/updownratiodialog.h \
     $$PWD/utils.h
@@ -87,6 +89,7 @@ SOURCES += \
     $$PWD/mainwindow.cpp \
     $$PWD/optionsdialog.cpp \
     $$PWD/previewselectdialog.cpp \
+    $$PWD/private/tristatewidget.cpp \
     $$PWD/raisedmessagebox.cpp \
     $$PWD/rss/articlelistwidget.cpp \
     $$PWD/rss/automatedrssdownloader.cpp \
@@ -121,6 +124,7 @@ SOURCES += \
     $$PWD/transferlistmodel.cpp \
     $$PWD/transferlistsortmodel.cpp \
     $$PWD/transferlistwidget.cpp \
+    $$PWD/tristateaction.cpp \
     $$PWD/uithememanager.cpp \
     $$PWD/updownratiodialog.cpp \
     $$PWD/utils.cpp