Start porting away from PlasmaCore.DataSource and fix some other problems

[ArrayBolt3]

Note that I may have made all these changes to the wrong branch, in which case I'm happy to port them to a different branch, test, and re-submit the PR.

Full changes list:

• Replace the "executable" engine in PlasmaCore.DataSource with a new ShellEngine system written in the C++ part of the code. (Fixes https://github.com/kfocus/kfocus-source/issues/10)
• Add two missing runtime deps.
• Be consistent in how we obtain error messages from kfocus-fan-set and kfocus-power-set. (Previously we used a string split to chunk the shell script messages into usable parts in one place, but used substrings in another place since the error message in the other place had a colon in it. Now instead we use substrings in both places.)

[ArrayBolt3]

Hmm, I see my commit message is no longer right on the ShellEngine commit where it says "and fix some other problems". I'll go back and fix that.

edit: nevermind, that was my PR title I was looking at. GitHub is confusing.