mime-settings: Properly quote command parameters

Like 85! just for the 4.16 branch.

I only swapped xfce_str_replace and xfce_str_is_empty by exo_str_replace and exo_str_is_empty, since xfce_* did not exist for 4.16.

@Tamaranch, since your name is on it, better take a look before I push it !

Edited by Alexander Schwinn

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