EXT-2493 FIXED Added strafe buttons.

- Added slide left and Move right buttons to movement actions panel, when inserting them in xml organized buttons in panel into 3 columns to make their position more easily configurable.

- Removed hiding of fly up and down buttons. When not flying(in walk/run mode) they act as jump ans crouch.

- Added movement mode(walk/run/fly) dependent tooltips for fly/jump/crouch and slide buttons.

- Added comment regarding implementation of move floater buttons toggling when user moves via keyboard (because it's not quite obvious and time consuming when you first encounter it) to LLFloaterMove::postBuild().

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/360

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branch : product-engine