Reorganize and unify naming for all reactive operators

[glopesdev]

The current namespace organization is messy for nested reactive operators, as they are split between Bonsai.Expressions and Bonsai.Reactive. It would be more clear to have everything that is not purely expression manipulation inside Bonsai.Reactive. We can also drop the Builder suffix for all nodes which are moved.

Finally, for consistency we might want to rename operator variants with a suffix, rather than prefix (e.g. WindowCount instead of ElementCountWindow).