[TYPO3-core] Extension dependencies of T3D exports

[Ingmar Schlecht]

Hi guys, mainly Karsten,

since rc2, the impexp tool automatically tries to load extensions which
T3D files depend on. Unfortunately, there are still some issues with
that feature.

Karsten, please test it with the quickstart T3D from
http://webempoweredchurch.org/fileadmin/proj/misc/ which will be
announced together with 4.0.

The issues are:
 - It doesn't check wheter extensions are *loaded* but whether they are
   *present on the server*. So impexp basically says "Everything's OK,
   proceed to importing" even if the extensions are not loaded but
   only available to be installed.
 - If you click one of the "+" (=>install extension) icon, you get
   thrown out of the process. Those "+" icons shouldn't be displayed at
   all, because the user doesn't have to click the "+" but just the
   submit button at the end of the page

Karsten, can you fix this before 17:30 (at that time Stucki and me want
to start packaging rc3).

cheers,
Ingmar