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.
It was only used to share implementation between IconButton and Link.
However, there was only one non-trivial method, handleTS(), and that
method used a different code path for each use case: doubleClick was
always false for IconButton and always true for Link. So the total
amount of code was actually reduced by eliminating this code sharing.
The main motivation for this split is that I can now freely refactor
Link without having to worry about IconButton.
Each part of the code deals with either Links or IconButtons, but
not both: the base class is only used to share implementation and
not interface. Make this explicit by doing private inheritance.
