java: Improve build rules
java: Improve build rules

* Fix builds in subdirectories:
  * Add srcdir to Manifest.txt.
  * Remove srcdir from piccolo2d-core-3.0.jar and piccolo2d-extras-3.0.jar.

* Add dependency of SCROLLVIEW_CLASSES on SCROLLVIEW_LIBS.
  The SCROLLVIEW_LIBS are now automatically fetched when needed.

* Add .PHONY target for fetch-jars.

* Improve rule for clean target.

Signed-off-by: Stefan Weil <8d4c780fcfdc41841e5070f4c43da8958ba6aec0@weilnetz.de>

jbarlow83/tesseract,stweil/tesseract,jbarlow83/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,jbarlow83/tesseract,UB-Mannheim/tesseract,amitdo/tesseract,amitdo/tesseract,stweil/tesseract,stweil/tesseract,UB-Mannheim/tesseract,jbarlow83/tesseract,stweil/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,stweil/tesseract,amitdo/tesseract,jbarlow83/tesseract,amitdo/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract