Introduce construction/extension required properties

[medikoo]

/cc @kamsi

It should apply to both creating new classes (extensions), and creating new instances.

Currently we have just required setting, and all it does it prevents deletion of already set properties that have required set to true.

We need a mean, through which initialization of new object (or class) will crash when some properties are not provided to constructor (still at this point I'm not sure what should be the name of property that will mark that)

[kamsi]

@medikoo requiredByDefinition perhaps?

[medikoo]

Or betterrequiredByDefinition ;-)
I'm not sure does it say more than required (of which you can also say required by definition).

Probably constructionRequired would be nicer, but I'm not convinced by that name either

[mtuchowski]

mandatory ?

[medikoo]

@mtuchowski I had that in mind as well for a moment, but thought that it sounds just as an alias for `required, still indeed it may be good pick :)