WebShortcuts Runner: Refactor foreach, remove unnecessary method

Summary:
The foreach has been refactored and the loadSyntaxes method gets now called directly.
Additionally the cached (failed) key/provider can be reused for multiple sessions and is only cleared when the config changes.

Test Plan: Should compile and open urls as before.

Reviewers: ngraham, broulik, meven, apol

Reviewed By: broulik, meven

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27566