EXT-8501 FIXED Check menu items always looked as checked.

Reason:
Check menu items (instances of LLMenuItemCheckGL) had used LLUICtrl::getValue() to know whether they should draw the checkmark.
Recently this was broken when getValue() was overriden in LLMenuItemCallGL to return a string.

Fix:
Overriden getValue() in LLMenuItemCheckGL to return a boolean value, not a string.

Reviewed by Mike Antipov at https://codereview.productengine.com/secondlife/r/821/

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branch : product-engine