Add commands `\mathrel` and `\joinrel`

[rstm-sf]

Add commands \mathrel and \joinrel.

This will be needed to replace the washout command \bowtie, as it is replaced by the following command (https://tex.stackexchange.com/q/100603):

\mathrel \triangleright \joinrel \mathrel \triangleleft

Now \bowtie is defined in PredefinedTexFormulas.xml as follows

<Formula name="bowtie" enabled="true">
  <CreateFormula name="f">
    <Argument type="string" value="\triangleright" />
  </CreateFormula>
  <MethodInvocation name="SetFixedTypes" formula="f">
    <Argument type="AtomType" value="Relation" />
    <Argument type="AtomType" value="Ordinary" />
  </MethodInvocation>
  <CreateFormula name="g">
    <Argument type="string" value="\triangleleft" />
  </CreateFormula>
  <MethodInvocation name="SetFixedTypes" formula="g">
    <Argument type="AtomType" value="Ordinary" />
    <Argument type="AtomType" value="Relation" />
  </MethodInvocation>
  <MethodInvocation name="AddStrut" formula="f">
    <Argument type="Unit" value="Mu" />
    <Argument type="double" value="-1.8" />
    <Argument type="double" value="0" />
    <Argument type="double" value="0" />
  </MethodInvocation>
  <MethodInvocation name="Add" formula="f">
    <Argument type="Formula" value="g" />
  </MethodInvocation>
  <Return name="f" />
</Formula>

Note: Maybe this will help #215?