Blender Git Loki

File Browser: Refactor file "UUID" code (which wasn't really a "UUID")

To some degree these are changes in preparation of further Asset Browser
related changes, see D11119. But also, the current UUID design was written for
the old Asset Engine design, which isn't part of the current Asset
Browser/System design anymore.
And lastly, "UUID" are a well established standard
(https://en.wikipedia.org/wiki/Universally_unique_identifier) which this
implementation didn't follow. What we have here is more of an index, or a
unique identifier (https://en.wikipedia.org/wiki/Unique_identifier).

So this does the following changes:
* Renames "UUID" to "UID"
* Changes the type of the UID to (a typedef'ed) `uint32_t`, which is more than
enough for our current asset system design and simplifies things.
* Due to the new type, we can avoid allocations for hash-table storage.
* Add/use functions for UID handling

Note that I am working on a major rewrite of the file-list code. Meanwhile we
want to keep things sensible.