Fix or silence compiler warnings

[SamVanheer]

Fix or silence compiler warnings. There are quite a number of them, though many are not relevant at this time.

Ideally no warnings should be silenced, but fixing all warnings is difficult since some of them require changes in how things are used, like enum class not supporting operator| by default.

In the short term warnings pertaining to buggy code should be fixed, other warnings should be fixed if it's simple enough, others should be silenced.