XML editor: improve handling of long attribute values
Alternative implementation of !1301 (closed).
The maximum length of attributes is large enough that the hard cut we make for performance reasons should rarely ever be visible (except for very large windows on very large screens). If it should happen nonetheless, match the gtk ellipsis by moving it to the end (instead of the middle).
Values with multiple lines are cut at the first newline with a visual indicator added indicating there's more content.
