Also cleans up margin and its hacks (alignment done with anchor/origin now).
Make osu! ruleset placements happen on mouse down
Fix editor crashing when entering with no beatmap selected
Fix TestBeatmap not setting appropriate ruleset ID
Rename editor tests namespace from "Editor" to "Editing"
Also makes `CreatePlayerRuleset()` and `CreateRulesetForSkinProvider()` not-null to avoid unwanted behaviour with their derivers