Mark methods/properties that return WinRT objects as nullable

[halildurmus]

Fixes #647

Note that we must also mark WinRT object parameters as nullable.

IStorageItemExtraProperties.RetrievePropertiesAsync(IIterable) Method's parameter can take null:

A collection that contains the names of the properties to retrieve. Pass null to retrieve all possible properties.

I'm thinking of doing that after we land this.

[timsneath]

Approved with clenched teeth :) Here's hoping that in the future WinRT adds null safety.

[halildurmus]

Approved with clenched teeth :)

Thanks!

Here's hoping that in the future WinRT adds null safety.

There was a proposal to introduce a NotNull attribute to the WinMD for this but apparently, they decided not to go with it (see microsoft/xlang#708) :(.