TableConvert: Splitting plugin a little up and adding Geany-file-type independent convert menu

This commit mostly is adding a special menu for converting something to a table without changing Geany's
file type settings. Currently it doesn't add much comfort with it, but it will allow in future also to
convert e.g. into different wiki syntax stiles.

However, during this work it appeared to make any sense to split plugin into more than just one huge c-file.