Before the code was modifying the format rect to compensate for space
being added or removed for the selection bar, but this should only
happen when the ECO_SELECTIONBAR setting bit changes.

The bug could be seen by inserting tabs stops 1cm, 2cm, & 10cm.  The
10cm tab stop would be inserted before 2cm since the comparison isn't
based on the numeric value.

Previously there was no such error, and if more than MAX_TAB_STOPS were
added, then some of the tab stops would be silently discarded.

Now that we no longer have a global Make.rules file, include them in 
each Makefile.

Reuse our caching mechanism (get_directory_contents()), but clear the 
cache if we have modified a directory's content.