Fix dynamic allocation of IconButton instances

Previously, IconButton instances to be added to button boxes were
allocated with new, but never freed with delete.

unique_ptr makes sure the buttons will be freed along with the button
box that owns them, or when code calls ButtonBox::clear.

The destructor of ButtonBox has been made redundant by this change, so
it's gone.
In C++11, it is no longer necessary to avoid ">>" in nested templates
Removed GMenu2X pointer from ButtonBox

It was only used to fetch resY, so I replaced that by taking the
y-coordinate as an argument. That is also more consistent with the
x-coordinate which was already an argument.

The x-coordinate was changed to a signed int, since that is the norm
for paint coordinates. It can be useful for drawing widgets that are
partially off screen, for example during a (dis)appear animation.

In InputDialog, the ButtonBox field was changed from a pointer to
a plain data member. There was no need to dynamically allocate it.
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.
Removed "delegate.h"

The abstraction it provided was so thin that I think it is simpler to
just have the code use std::bind directly.
Rearrange calls to GMenu2X::drawButton from left to right

This breaks up large reversed call chains in a few places.