Incorrect xpathparser key, when sorting the mappers.

After sorting mappers the xpath parser key is incorrect and may become duplicated. For example, let's say you have 3 xpathparser items and they are orderd which results in the following order:

1. xpathparser:0
2. xpathparser:2
3. xpathparser:1

With the current code the last key will be used, and 'xpathparser:' part is removed from it so we keep the value of 1 and then +1 to it to create the next key (for the mapping form select list). This will result in xpathparser:2 which as we can see above is already used.