Make the slider grab keyboard focus when clicked on
Make the slider grab keyboard focus when clicked on

This allows stepping by 1 when using an orthoslice on a large dataset.

mathturtle/tomviz,thewtex/tomviz,cryos/tomviz,cjh1/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,cryos/tomviz,cryos/tomviz,OpenChemistry/tomviz,cjh1/tomviz,cjh1/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,thewtex/tomviz,OpenChemistry/tomviz,thewtex/tomviz