Expose 'change colors' through dbus interface

Summary:
This exposes the Okular's functionality of changing the document's colors
through its d-bus interface. This is mainly useful for scripting Okular to
switch to a 'day mode' or 'night mode'.

[Example] Switching themes of two applications with one hotkey:

Reviewers: Okular, aacid

Reviewed By: Okular, aacid

Subscribers: okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D13471