Fix KUrl::isRelativeUrl to allow all RFC3986 characters in scheme

RFC3986 defines the following syntax for URI schemes
scheme = alpha *( alpha | digit | "+" | "-" | "." )

The '.' was missing which leaded to the problem that
xdg-open opc.tcp://localhost didn't work.

Background:
xdg-open calls kde-open which then uses KUrl and so leaded to this
problem. Of course this bug may affect also many other locations
using KUrl::isRelativeUrl.

This fix also adds a unit test to test the fix and avoid future
regressions.

Acked by dfaure

REVIEW: 129208