Rename Id type in src/id.rs

[nicoburns]

          `Id` is kind of an awful name for this type, although this PR isn't really the place to fix it

Originally posted by @DJMcNab in #166 (comment)

[DJMcNab]

To clarify, because the original comment wasn't constructive:

The type name tells you nothing about what the type is for (and the documentation also doesn't help). I also don't think this type is the identifier for "everything", i.e. both views and widgets, which might have justified this current name.

(Although I've not dug into the code to investigate this)

[DJMcNab]

The mentioned file is now gone, since #290

The ids are now ViewId in Xilem and WidgetId in Masonry.