Update ts: alternative to #5

[JirkaVebr]

This is a largely unfortunate duplicate of #5, although it, I believe, addresses the issues at hand slightly better.

Rationale comparing these changes to #5:

1. TS can be somewhat unpredictable at times, and so fixing its version to ^3.5.1 shoul be more stable, and thus preferable to ^3.5.x. We can always release new versions if need be.
2. The casts in Component.ts are strictly more specific than those in Upgrade TypeScript to 3.5 and fix compilation errors #5, and thus are slightly less of an unfortunate workaround.
3. The early return in InView.ts is more elegant than a large branch.
4. Albeit at the expense of an as any cast, the changes in inject.ts still rely on setting properties as opposed to attributes, which is the correct approach.

[enzy]

What about the custom HTMLElementEventMap interface. Why is it still used?

[JirkaVebr]

@enzy Well, it's a rather unfortunate workaround but it still has its place. The problem is that the official TS HTMLElementEventMap only includes events whose W3C specifications are candidate recommendations or further. However, that excludes focusin and focusout as these are still just editor's draft. Despite that, they have practically universal browser support, and thus our library adds them so that people can listen to them. Essentially, our extension will eventually get removed but I'm afraid the time has not come yet. 😐

[enzy]

Oh, it's this part <E extends keyof HTMLElementEventMap> I haven't noticed. Okay 👍

[enzy]

setting properties as opposed to attributes, which is the correct approach

→ because properties are typed, attributes are just strings 👍