[TASK] Add ExtJS3 Compatibility Layer Extension

Until all Backend-Modules are migrated to ExtJS4 we need to
make them available via ExtJS3. Thanks to Iframes this is possible.

This sysext ships all ExtJS 3 Code which had been in place before
the migration and adds an LoaderClass, which hooks into PageRenderer
to exchange the Library if requested.

Besides that, extjs3_compat is defined as required extension to be
automatically available.

This whole package will be reverted again until the final 4.7 release.

Change-Id: Iacbbaff2217c7b04232ac31773f613d3913abb36
Resolves: #32720
Releases: 4.7
Reviewed-on: http://review.typo3.org/7427
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader