Some paths in Blender support the `//` prefix for blend-file relative
paths however this is not communicated anywhere.
Support declaring this with a new flag which can be applied to filepath
& dirpath typed properties.
This is used to:
- Show red-alert when "//" are used in paths which don't support a relative prefix.
- Show a warning in the tooltip that the relative suffix is used when unsupported.
- Python warns if this prefix is ever used in an assignment or
function argument.
Resolves#133456.
Ref !137060
ab20edf469