MDL-39087 Simplify get_uninstall_url() interpretation

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).

lib/editor/tinymce/adminlib.php

@@ -35,6 +35,11 @@ require_once("$CFG->libdir/pluginlib.php");
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class plugininfo_tinymce extends plugininfo_base {
+
+    public function is_uninstall_allowed() {
+        return true;
+    }
+
     public function get_uninstall_url() {
         return new moodle_url('/lib/editor/tinymce/subplugins.php', array('delete' => $this->name, 'sesskey' => sesskey()));
     }