Avoid duplicate Control and Primary in modifier strings
Only insert the string for a modifier once, in case there is more than
one entry for it in the modifierList
, like there is for Control, which
is also Primary.
We also move Primary before Control to prefer the former in string representations.
This fixes the case where e.g. CCSM will write <Control><Primary>x
, which although working fine, really looks bizarre and makes it harder for a human to read (or a naive tool not using libcompizconfig).