Only add hot corner for primary and "top left" monitors

To avoid having hot corners that accidentally trigger when e.g. trying
to hit the panel on the primary monitor we add hot corners only to
monitors that are "naturally" top left (top right for RTL).

For instance, we'd like a hot corner here:

corner -> +-------------
          |            |
+---------+            |
|=========|            |
|         |            |
|         |            |
|         |            |
+---------+------------+

But not here:

          unexpected hot corner
          ↓
+---------+-------+
|=========|       |
|         |       |
|         +-------+
+---------+

https://bugzilla.gnome.org/show_bug.cgi?id=645116