add knewstuff support for color schemes

Summary:
schemes are supplied by Konsole Color Scheme category on store.kde.org

https://store.kde.org/p/1216368/

• new manager helpers to unload a theme and check a file's name validity
• new Get New.. button to start the KNS download dialog
• upon KNS completion we'll attempt to load all installed files as schemes
• for KNS entities which failed to load anything we'll show a warning as the theme is malformed
• for removal we'll first try to remove schemes through KNS to avoid its registry going out of sync with the on-disk "installedness" of an entity
• properly disable copying of the scheme manager (needs to be private)

FEATURE: 18.04.0
CHANGELOG: Support for downloading color schemes from the KDE store

Test Plan:
category is T8053 (can also use KDE Color Scheme KDE4 for testing)

• get new -> install scheme -> close -> shows up in list
• remove from list -> get new -> not listed as installed
• install random stuff from kde color scheme category -> close -> warning should be displayed
• install scheme -> get new -> remove scheme via dialog -> close -> not listed as installed

Reviewers: hindenburg

Subscribers: Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D10766