[IMP] web: add tooltip on Dropdown

This commit adds the possibility to add a tooltip on a Dropdown.

closes odoo/documentation#1396

Signed-off-by: FrancoisGe <fge@odoo.com>

content/developer/reference/frontend/owl_components.rst

@@ -268,6 +268,9 @@ being present in the DOM or not.
    * - ``hotkey``
      - string
      - hotkey to toggle the opening through keyboard
+   * - ``tooltip``
+     - string
+     - add a tooltip on the toggler
    * - ``beforeOpen``
      - function
      - hook to execute logic just before opening. May be asynchronous.