Summary: This patch largely rewrites the Message page to specifically talk about modal message dialogs, as contrasted with non-modal informational inline messages. Redundancy and confusion should be reduced as a result.

Test Plan: {F6486797}

Reviewers: fabianr, #kde_human_interface_guidelines

Reviewed By: fabianr, #kde_human_interface_guidelines

Differential Revision: https://phabricator.kde.org/D17656