slightly more sane and less repetitive fetching of engines

[Foxinatel]

Mostly just some code cleanup in this PR. The only real change in functionality is that we now fallback to the Kwin tiling engine, as that essentially just mimics standard behavior anyways. We also get to remove the slightly odd level of indirection whereby we push to the engines hashmap using a string in createNewEngine() and then have to use a non-null assertion in the outer function body to get the value back.

[zeroxoneafour]

Lgtm, I'm going to go back in later and private some of these methods to prevent TilingEngines from escaping their prison