This implements (most of) the proposal in #122743:
* Add a new `IDP_FLAG_STATIC_TYPE` IDProperty flag.
* Update `BPy_IDProperty_Map_ValidateAndCreate` and related to never
change an existing property type if statically typed.
The biggest change happens in bpy assignement code, since instead of
replacing the old exisitng property by a newly created one, and copying
over a few settings, now the old property is kept if possible, and a new
one is only created if needed.
And in case the existing property is statically typed, if it cannot be
re-used to store the given value, and error is reported and it remains
unchanged.
`IDP_ARRAY` is also supported for basic numeric types, so 'vector'
properties and such work as expected. Lentgh is considered as part of
the static type (i.e. one can only assign a 3 components py sequence to
a 3-len array property, etc.).
Such in-place update is not yet implemented for `IDP_IDPARRAY` and
`IDP_GROUP` types. While important (especially the group one), they are
not that critical for the current issues related to changing IDProperty
types.
Bastien Montagne
4d1fe98604