[[ Bug 12786 ]] Treat XK_ISO_Left_Tab the same as XK_Tab.

On Xorg systems, a \<Tab> keystroke has keysym XK_Tab, but a
\<Shift>+\<Tab> keystroke has keysym XK_ISO_Left_Tab (with the shift
modifier bit set).

Modifying the translatekeysym() function to translate XK_ISO_Left_Tab
into XK_Tab ensures that \<Shift>+<Tab> key combinations are handled
correctly.