Blender Git Loki

RNA: Allow structs to define tags for their properties

Adds support for defining a number of tags as part of the rna-struct
definition, which its properties can set similar to property-flags.
BPY supports setting these tags when defining custom properties too.

* To define tags for a struct (which its properties can use then), define the tags in an `EnumPropertyItem` array, and assign them to the struct using `RNA_def_struct_property_tags(...)`.
* To set tags for an RNA-property in C, use the new `RNA_def_property_tags(...)`.
* To set tags for an RNA-property in Python, use the newly added tags parameter. E.g. `bpy.props.FloatProperty(name="Some Float", tags={'SOME_TAG', 'ANOTHER_TAG'})`.

Actual usage of this will be added in a follow-up commit.