Add tool tips to column headers

[misje]

It is currently possible to add a description to columns types (ColumnType), but I believe this field is unused (although I haven't looked through all the code). The web interface already utilises tool tips in several locations. What about displaying the column type description in a tool tip when hovering the column headers?

Since columns are dynamic and may change with each row, having the ability to quickly get a hint on how to interpret data in a rarely seen column would be great. I am happy to populate this information when writing artifacts, because I really value well-documented artifacts.