[mcetool] Add dimming demo mode. Fixes MER#1542
[mcetool] Add dimming demo mode. Fixes MER#1542

There already is "keep display on" demo-mode that disables all timer based
display dimming and blanking. Often it would be more useful to allow
dimming and disable only blanking timers.

Add "mcetool --demo-mode=dim" option variant.

mlehtima/mce,mlehtima/mce,spiiroin/mce,spiiroin/mce,maikelwever/mce,spiiroin/mce,spiiroin/mce,mlehtima/mce,maikelwever/mce,maikelwever/mce,mlehtima/mce,maikelwever/mce