gdm: disallow cancel after verification succeeds

Once verification has succeeded, the train's already
left the building and we shouldn't allow canceling.

This commit renders the cancel button non-reactive
and makes the cancel function be a noop after
verification succeeds.

https://bugzilla.gnome.org/show_bug.cgi?id=740141
gdm: fix sensitivity of auth prompt when cancelling early and user list is disabled

If the user list is disabled and the user clicks cancel quickly enough
after typing their username, they can get in a state where the
auth prompt gets stuck in the insensitive state.

This is because the login dialog code makes the prompt insensitive
while while pam is processing the provided username, but the prompt
only makes itself sensitive again when it is hidden.

This commit makes it sensitive right before asking for a username again.

https://bugzilla.gnome.org/show_bug.cgi?id=740141
St: support css margin property

It's implemented similar to the padding property, but instead of taking
into account the margin values at drawing time in node-drawing, we set
the clutter actor margins in StWidget when the style is computed.

In the case that a CSS margin is not specified, we don't to set a value
of 0 to the clutter actor margin. In this manner it allows to use
Clutter margin values set in the code. However, the margins that are set
both in the code and in the CSS on the same side, the result is
unpredictable.

We avoid to set the clutter actor margin values to 0 if there's no CSS
margin values defined, so we still allow clutter actors to use margin
set in the code.

https://bugzilla.gnome.org/show_bug.cgi?id=728437