Improve LLUICtrlFactory's performance

[akleshchev]

LLUICtrlFactory is widely used yet uses performance unfriendly variant of a singleton, swap to simpleton.

Code task that branched off from async inventory.

Test plan

• There is no test for this specific change. Inventory performance testing will be covered by https://github.com/secondlife/viewer-private/issues/595

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[akleshchev]

Swapped LLUICtrlFactory from llsingleton to simpleton. This should make any UI creation sligltly cheaper.

This is mostly an offshot of 'async inventory'. Bbecause 'async inventory' was trying to optimize the same thing in there but for inventory specifically. I just wanted the 'does it really belong there' argument out of the road of async inventory and not clog the review. No specifc test plan exists, inventory is potentially the most affected, but performance boost is debatable.

[Dan-Linden]

Passed QA in the scope of secondlife/iqa#660
No specific test, so not tested.