Blender Git Loki

Add custom property support for assets

I didn't do this earlier because I was afraid it would make reading of asset
data (and asset data only!) difficult. Mainly because custom properties could
store pointers to other data, that would have to be read too then (e.g. an
object). But turns out, we can easily disable custom pointers to other
data-blocks, so reading stays simple :).

Merge branch 'master' into asset-metadata

Fix crash loading some files after recent merge

Caused by badly resolved merge conflicts.

Merge branch 'master' into asset-metadata

Merge branch 'master' into asset-metadata

Merge branch 'master' into asset-metadata

Merge branch 'master' into asset-metadata

Fix compile error after last merge

Merge branch 'master' into asset-metadata

Merge branch 'master' into asset-metadata

Fix mistake in previous commit

The "Make Asset" button in context menus wouldn't actually do anything.

Show "Make Asset" in the context menu of buttons showing IDs

Fix build error after latest merge

Merge branch 'master' into asset-metadata

Merge branch 'master' into asset-metadata

Fix compile error after last merge

Merge branch 'master' into asset-metadata

Adapt to new file read/write design from master

Partially a compile fix for previous commit. But also moving asset-data
read/write into `asset.c`, like we do it for more and more
data-structures in master.

Merge branch 'master' into asset-metadata

Follow up to previous commit, rename asset operator in Outliner

Forgot to rename it in the Outliner as well ("Create Asset"->"Make
Asset").