[ux] Make tag combobox in symbol selector non-editable

There doesn't seem to be any reason to support edits here!

Author: nyalldawson

src/ui/symbollayer/widget_symbolslist.ui

@@ -100,7 +100,7 @@
         </size>
        </property>
        <property name="editable">
-        <bool>true</bool>
+        <bool>false</bool>
        </property>
       </widget>
      </item>