Make some static text in GTK dialogs selectable.

Make some static text in GTK dialogs selectable.

I've made the licence text, the About box, and the host key dialog
into GTK selectable edit controls. (The former because it contains a
lot of text; the About box because pasting version numbers into bug
reports is obviously useful; the host key because of the fingerprint.)

(cherry picked from commit 21101c7397e460933635a7bfed813864fc4f88fe)

Conflicts:
	unix/gtkdlg.c
	unix/unix.h

(cherry-picker's notes: not a trivial resolution, since I had to apply
the equivalent changes in the pre-GTK3-port version of the code)