x11: improve tab selection a lot
x11: improve tab selection a lot

DrXyzzy/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,tscholl2/smc