sysutils/qtpass: improve port

• install the icon for qtpass
• install qtpass-desktop file
• drop buildtools and linguisttools from the runtime dependencies
• instead of defining a do-install target, fix qtpass.pro to work with the ports system [that change is probably up for debate...].

PR: 204122
Submitted by: Tobias C.Berner <tcberner@gmail.com> (kde)