Validate plugin PRIORITY and VERSION fields upon loading

[Tieske]

Adds stricter checks on plugins when loading. Priority must be a valid integer (not inf nor NaN), and the version must be parseble as x.y.z.

This means the pre-functions plugin now has a PRIORITY = 1000000, which previously was +inf.

review: best reviewed without whitespace changes

[fffonion]

@Tieske Does this mean all external plugins needs to have Version and Priority defined?

[Tieske]

Does this mean all external plugins needs to have Version and Priority defined?

Yes, so it's a breaking change. But it's for the better imho since it allows for better control of plugins, they should have it anyway. Only thus far they got away without it.