This gives us 9 flags available again for properties (we had none anymore),
and also makes things slightly cleaner.
To simplify (and make more clear the differences between mere properties
and function parameters), also added RNA_def_parameter_flags function (and
its clear counterpart), to be used instead of RNA_def_property_flag for
function parameters.
This patch is also a big cleanup (some RNA function definitions were
still using 'prop' PropertyRNA pointer, etc.).
And yes, am aware this will be annoying for all branches, but we really need
to get new flags available for properties (will need at least one for override, etc.).
Reviewers: sergey, Severin
Subscribers: dfelinto, brecht
Differential Revision: https://developer.blender.org/D2400
