kcm: Set new quality default to 75, minimum quality to 50

The qualities below 50 are rather poor and any gained responsiviness does not really help when you can't see what you're doing.

Instead, set the minimum quality value to 50, where it's still legible. Set default quality to 75, which is around middle of the slider, and should work well for most cases.

Note that users can still run krdpserver --quality 1 if they want to experience what it's like to use your computer through plate of porridge.