The get_uninstall_url() method of all subclasses of plugininfo_base
class is now expected to always return moodle_url. Subclasses can use
the new method is_uninstall_allowed() to control the availability of the
'Uninstall' link at the Plugins overview page (previously they would do
it by get_uninstall_url() returning null). By default, URL to a new
general plugin uninstall tool is returned. Unless the plugin type needs
extra steps that can't be handled by plugininfo_xxx::uninstall() method
or xmldb_xxx_uninstall() function, this default URL should satisfy all
plugin types.
The overall logic is implemented in plugin_manager::can_install_plugin()
that respects the plugininfo class decision and vetoes it in certain
cases (typically when plugin or its subplugin is required by some other
plugin).
David Mudrák