(GEP-113) Add deprecation warning for use of import.

This commit adds a new 'ImportIsDeprecated' validation issue and
code that will report when the import keyword is used. The severity
can be controlled by a validation preference which is set to 'warning'
by default for target platforms below 4.0 and hard-coded to 'error'
for 4.0 and above.
(GEP-113) Add manifestdir preference

This commit adds both a global and a project specific setting for
the preference "Manifest Directory". The setting can then be used
in the "Search Path" as $manifestdir. The default search path
is changed to include '$manifestdir/*' instead of 'manifests/*'.