Add acquireFocusOnMouseDown prop to Focusable, set to false in LayerFace

[baaygun]

When the name of the layer in a LayerFace was clicked on, we would immediately call OS.acquireKeyboardFocus due to the mousedown listener in Focusable mixin (which is the mixin of TextInput showing layer name). But we would never call OS.releaseKeyboardFocus because those were only called on commit/cancel/finish methods.

For TextInputs like layer name, we don't want to acquire focus unless the user double clicks to start editing. We instead acquire focus on _beginEdit function which is what happens when user double clicks.

Addresses #3581

[iwehrman]

Do we need to explicitly call acquireFocus once the TextInput becomes editable?

[baaygun]

No @iwehrman, _beginEdit calls acquireFocus. It is called by both click, focus and doubleclick based on different scenarios.

[iwehrman]

Roger. In that case: looks good!