[timeline] Canonicalize Url

Summary:
In case someone removes everything but the leading "timeline:" scheme
from the Url, both "today" and "calendar" are listed, but selecting
any results in e.g "File or folder timeline:calendar" does not exist,
as the correct Url is "timeline:/calendar".
Canonicalize the Url and redirect when the cleanup had any effect.

Test Plan: dolphin timeline:

Reviewers: Baloo, Frameworks, poboiko, ngraham, astippich

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20101