Retain color history

This patch adds capability to remember the recent color history in both saved documents (as long as they are saved in Krita native format, .kra) and as a global setting (that provides default values for new documents and serves as a persistent history for documents not in .kra format, like .png or .gif). After this patch the history becomes per document but shared between multiple views of the same document. As some reviewers thing the total shared history may be an advantage for some workflows, a configuration checkbox may be added later.

This extension has been originally proposed for Krita as request 1424, in response to the bug report 357493.

The request ran into problems after the reviewer opinions diverged, some calling for the history per document and others still preferring the global history. While the compromise has been proposed that by idea should be good for both sides, this one was then criticized for pure architecture and the thread of "subtle bugs".