The code was previously getting the same text in the loop from the
editor, and it was converting each of the prefixes to compare against
for each URL candidate.

Before a single length was used for the number of characters to retrieve
from the text, and to keep track of the size of the buffer.  These are
not equivalent, since there is a possible end of line conversion.

Previously the only convenient way to get the start and end of the
selection was through offsets, which eventually need to get converted
back into items in the linked list storing the text.  The new function
will help with eliminating these inefficiencies.

This function will make it easier to work with ME_Cursor objects, which
should be used in a lot of places instead of character offsets (which
often require seeking through the linked lists to perform operations
with).