Before this commit, menus in navbar and clickable elements on the home
menu were not openable in a new tab through a ctrl-click or through the
middlemouse button.
In order to achieve that, we now use default behavior of <a href/> elements.
As such default behaviour would conflict with <DropdownItem/> clicks,
a global click handler is added in the WebClient constructor which stops
the click event propagation under these specific circumstances
(ctrl-click inside an <a href/> element).
Besides this work, it has been found that the <Dropdown/> root element
should not always be a <div/> as it was, but sometimes it has to be
another HTML element in order to comply with the W3C specs. E.g. a
<Dropdown/> component as a first level child of an <ul/> element must
have a <li/> root element instead of a <div/> one.
Thus, a new prop is added to the <Dropdown/> component (`tag`) allowing
the developer to choose another root element: `<Dropdown tag="'li'"/>`.
closesodoo-dev/odoo#913
Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>