Previously, only DOM nodes with `is_editable()` allowed selection via
the mouse. This had the unwanted consequence, that read-only
input/textarea elements did not allow selection.
Now, `EventHandler::handle_mousedown()` asks the node's non-shadow
parent element over the new virtual method `is_child_node_selectable()`,
if selection of the node is allowed.
This method is overridden for `HTMLButtonElement` and
`HTMLInputElement`, to disallow selection of buttons and placeholders.
Fixes#579
6c9adf3dbc