Fix browse for files/folders doesn't enable "Compare" button.

Since KUrlComboBox::setUrl doesn't emit textChanged (by design according
to it's documentation) we need to emit a signal from FilesPage when
we set the url programatically so the kompareurldialog knows something
changed and to enable the button if both boxes have content.

BUG: 390024

(cherry picked from commit 006f36d3)