Chart matrix updates

This pull request has tree commits. There are some of changes, here's the most important:
The first adds a new matrix that can handle strings in the cells as well as numbers, it also adds the possibility to use row and/or column header in the matrix editor.
The second commit is about the chart. It will add the possibility to add point labels in the chart. But the main difference are that the chart interacts with both the matrix editor and the MolTableEditor, so that a selection in the chart will select the corresponding cells in the other editors and vise versa.
The third commit just adds some documentation (i.e. some help pages) and cleans up the code a bit.