relativePathRow: Fix regular expression for validation

flatpak-override --persist option can take basically any
string, so there isn't much to validate here, except one
string that can break the config files, e.g. linebreaks.

Closes #558

src/widgets/relativePathRow.js

@@ -54,7 +54,7 @@ var FlatsealRelativePathRow = GObject.registerClass({
     }
 
     _setup() {
-        this._expression = new RegExp(/.*\S.*/);
+        this._expression = new RegExp(/^[^\n]*$/);
 
         this._entry.connect('notify::text', this._changed.bind(this));
         this._button.connect('clicked', this._remove.bind(this));