Update snippets to use modern language features (eg "nameof")

[ljw1004]

The "propdp" snippet spits out a dependency property. I think it should spit out the nameof(.) operator, rather than spitting out a text literal.

[Pilchie]

As with #3469, we should probably do a sweep through all snippets. One consideration from the past is whether we realistically expect customers to use these snippets in scenarios where they would set their "langversion" switch back, since we don't have a mechanism to do different things in snippets.

[miloush]

This snippet is actually one of the worst from the very beginning. Apart from nameof,

• you have to delete the comment every single time
• it should use ClassName() to fill in the ownerclass
• does not support specifying derived metadata
• is cumbersome to use for declaring multiple properties consequently

plus my personal preferences do not improve it,

• I put the static declaration on a single line
• I put static members before instance ones

(and snippets for read-only DP and attached DP are missing completely)
I have submitted Connnect feedback about that long time ago but that seems to be gone now.

As for "langversion", snippets has <Function> thing that [I think has great potential but is underdeveloped and] could potentially handle it if needed.

Could perhaps the snippets be put on GitHub or somewhere were people could contribute to them?