Using markers

The example shows how to obtain specific coordinates in a (La)TeX text. This
allows to mark specific portions of text, setting arrows, braces and many
more. A marker can be set with the command `\PyXMarker`... which takes the
name of the marker as an argument (`id` in the example code). The position of
the marker can then be obtained from the textbox, which is returned by the
`canvas.text` method.