It's not always safe to access pixel data of a window's backing store
since the kernel may decide to purge it at his leisure. Fix this by
instead picking colors from the color spectrum bitmap directly instead.
Also fix up mouse event logic while we're here so it only cares about
the left mouse button
Fixes#1657.
Now that add() returns a WidgetType&, we can't rely on the parent of a
GUI::Dialog to still keep it alive after exec() returns. This happens
because exec() will call remove_from_parent() on itself before
returning.
And so we go back to the old idiom for creating a GUI::Dialog centered
above a specific window. Just call GUI::Dialog::construct(), passing
the "parent" window as the last parameter.
Since the returned object is now owned by the callee object, we can
simply vend a ChildType&. This allows us to use "." instead of "->"
at the call site, which is quite nice. :^)
