Add an option to ignore numeric values

An option to threat numeric values as just text would be helpful. It could be represented by offering a `none` option in the _decimal_ section of the footer bar.
I came across this issue when I had to update a few line in a huge tab-separated file. The workflow should have been:
- processed the few line in numbers
- export to csv
- open with TableTool
- convert with appropriate settings

Since I wanted to compare the original and the new file with a diff tool I had to go back and replace the decimal point in all numbers to an arbitrary character so that TableTool didn't interpret them as numbers. Then make the conversion and replace the placeholder character back to the decimal point. Otherwise TableTool was truncating the decimal values (removing trailing zeroes) and the diff would highlight the whole file.

Start by locating the footer bar's decimal setting and the code that interprets numeric values during conversion. Implement the proposed `none` option so values are treated as text and trailing zeroes are preserved, then verify the CSV conversion workflow against a diff of the original and exported files.

Written by the indexing model from the issue text.