EXT-8385 FIXED Fixed simultaneous availability to click "Sit here" and "Stand Up" on object.

Made only one of them enabled in the same time, didn't make disabled one invisible because it was decided so in EXT-8094.

- This bug is the result of EXT-8094 fix. Corrected logic of enabling "Sit here" and "Stand up" both in xml and code.

- Removed class LLObjectEnableSitOrStand because it's not needed anymore, and it's handleEvent() method duplicates is_object_sittable() function.

- Renamed Object.EnableSit to Object.EnableGearSit because its name is now used for enabling object menu sit, and this "simple" version is used only from object inspector's gear menu.

Reviewed by Mike Antipov at https://codereview.productengine.com/secondlife/r/761

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branch : product-engine