Preferences: Duplicate string gvalues (#70)

Based on Xfwm4 commit:
xfce/xfwm4@056ba6ad

The gvalues strings were considered statics, hence not duplicated.

As the caller frees the strings after setting the gvalue, the actual
content would end up pointing at free data.

Consider string gvalues as regular strings, meaning that the given
string value is duplicated as a gvalue.
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