Add commands `\mathrel` and `\joinrel`

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):
```tex
\mathrel \triangleright \joinrel \mathrel \triangleleft
```

Now \bowtie is defined in PredefinedTexFormulas.xml as follows

```xml

```

Note: Maybe this will help #215?