Fix auto-selection of URL field with X11 selection

We use `focusin` event instead of `click` because when using the latter,
when the selection is created it isn't put into X11 PRIMARY selection.
And as a Linux user I'm very used to such behaviour that when something
gets selected, it's ready for pasting, which is the primary purpose of
the clone URL field.

When `focusin` is used, the selection is copied to X11 buffer
automatically, so it works as expected (tested in Firefox and Chrome).