ui: add form/Tag

ui: add form/Tag

displays a single tag, with the ability to edit inline on click (when
the mode is set to editable). This brings up a list of globally available tags
for simple selection.

this is a basic ext component, with 'i' tags for the icons (handle and
add/remove button) and a span (for the tag text)

shows the tag by default, and if put in editable mode, makes the
span editable. when actually starting the edit, shows a picker
with available tags to select from

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>