Add api to ellipsize labels to multiple lines

When setting the lines property, the label will be ellipsized
to that many lines, with the ellipsis only appearing in the
last line. This is different from how ellipsization of multi-line
labels normally works in GTK+.
testsuite: Add a clipboard test

I'm not sure I like it yet because it deadlocked once and I haven't
figured out why that is.
label: Fix gcc warning
label: Initialize lines value to -1

Fixes defaultvalue test.