Pass action to IconButton constructor

This removes the need for a separate setAction method.

The default action is the empty action, which does nothing. However,
a touch event on a button with the empty action is no longer considered
handled.

Menu::btnContextMenu was changed from a unique_ptr to a plain data
member.

src/menusettingrgba.cpp

@@ -30,7 +30,6 @@
 using std::string;
 using std::stringstream;
 using std::unique_ptr;
-using std::move;
 
 constexpr unsigned int COMPONENT_WIDTH = 28;