Migrated all projects to packagereference

[Barsonax]

Solves #736

After this the nuget and netstandard branch need to get updated again (should make the changes in those branches alot smaller though).

[ilexp]

Nice 👍 Will review and merge soon-ish.

Note to self, things to check:

• Does it compile fine "on my machine" in addition to yours + AppVeyor after fresh checkout?
• Do a brief editor install check via local package repo, just in case.

[Barsonax]

Took the liberty of rebasing the netstandard branch and nuget branch in advance on this branch to solve any merge conflicts.

These branches need to be rebased again once this PR is merged to master (should be easy since all conflicts are fixed already).

See #740 and #741 for comparison/discussion.

[ilexp]

Took a clean / rebuild on my previous checkout to get rid of some dependency errors, in case anyone runs into this as well - compiles just fine from scratch though. Also did a quick package build and test install in a local repo and it looks like nothing changed on the binary output / compatibility side, as expected.

Merging this.