EXT-7104 WIP Enable Build button and set necessary callbacks.

Because it is not enough to call showInstance for Build Floater it was implemented in a such way:
* set save_visibility==true in floater tools to update button toggle state properly and subscribe Build button to listen visibility changes
* added callback to observe parcel changing to set build button enabled/disabled.

KNOWN ISSUE:
if build permissions are changed button will not change state until restart/teleport

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/413/

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branch : product-engine