Added a hardcoded string category collection

This is because the category and name pairs in the *.tdbl is not up to
date with the actual category + rdbIds. Thus, there can be more category,
rdbId pairs than there are category and name pairs. Since we do not want
to rely on id's to glance over the file names, we give them hardcoded
canonical names
Changed extension for string collections to .dat and to .xml when using
the RdbStringDataExporter
Removed redundant imports
Fixed an issue where all string collections were not exported
Changed signature of RdbStringFileReader.getStrings() to a Vector of int,
String tuples instead of int, Array[Byte]. The Vector is also filtered to
not export empty strings