LN_GLGUI: Fixed a few bugs with label.
LN_GLGUI: Fixed a few bugs with label.

- Fixed clicking wrapped label when 'label was #f or empty string.
- Fixed pressing backspace when cursor at left end of label.

emacstheviking/lambdanative,VincentToups/lambdanative,emacstheviking/lambdanative,VincentToups/lambdanative,emacstheviking/lambdanative,VincentToups/lambdanative,emacstheviking/lambdanative,emacstheviking/lambdanative,VincentToups/lambdanative,VincentToups/lambdanative