Revert hidden EXML feature "upper case EXML elements".
This saves one generated XSD per package and thus leads to more
light-weight EXML module artifacts and to a more concise set
of XSDs to complete in IDEA.
Nobody can have used the feature, anyway, since it is totally
undocumented, and Jangaroo 3 MXML is a much better approach.
The plan to get rid of config classes in EXML has been abandoned.
fixes JOO-73
EXML validation may fail even in validationMode "warn"
http://jangaroo.myjetbrains.com/youtrack/issue/JOO-73

Re-declaring config options in a subclass (even with the same
type) led to an invalid XSD being generated, which in turn,
instead of logging a warning or an error, threw an exception.
Fixed filtering of duplicate config options: even after fixing
a bug that caused fully qualified config attribute types parsed
from an ActionScript file to be incorrect, there still may be
inconsistencies in the type representation--fully qualified
versus imported--that lead to non-equal types, so for the
time being, ConfigAttributes (to be precise: all TypedFields)
no longer have to have the same type to be considered
equal, but only the same name.
We now use these "cleaned up" config attributes not only
when generating the XSD, but also In the config class template,
so that no longer property accessors are generated that are
red without an "override" keyword.
Even if something still slips through that leads to an invalid
XSD, such errors are no longer re-thrown, but logged as
configured (warning or error).
All EXML validation messages now start with "EXML validation:"
so that they can easily be extracted by tools.
fixes JOO-74
jangaroo-maven-plugin warns about invalid source path "src/main/joo-api"
http://jangaroo.myjetbrains.com/youtrack/issue/JOO-74

Simply check for existence before adding src/main/joo-api
to the compiler's source path.