Move AWT UI code to new org.eclipse.jgit.ui bundle

This new UI bundle contains the org.eclipse.jgit.awtui package,
which was moved out of the org.eclipse.jgit bundle.

org.eclipse.jgit.pgm depends on org.eclipse.jgit.ui, so we need
to update the classpath and make_jgit.sh to include it.

This move takes the awtui classes out of the Maven build, which
means we are no longer able to distribute these classes to our
downstream Maven customers.  The entire Maven package structure
needs to be overhauled so that Eclipse bundle matches 1:1 with the
Maven artifact.

Bug: https://bugs.eclipse.org/291124


Change-Id: Ibf1a9968387e3d11fdce54592f710ec4cc7f1ddb
Signed-off-by: Mykola Nikishov <mn@mn.com.ua>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>