cnf: doesn't need a JRE on the buildpath (no code)

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
ToggleNatureAction: only allow toggling on java projects

bnd projects are java projects

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
ToggleNatureAction: move into core

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Fix #428: 'add project nature' doesn't setup headless build files

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>