Add proof-of-concept compile time code generator

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Generate sources even in Eclipse, but only with a full build

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Generate the sources into target/generated-sources/from-template/

It is much better not to conflate generated sources with non-generated
sources. We employ the 'add-sources' goal of the build-helper plugin.

For Eclipse, happily, m2e already has support for imitating the workings
of the build-helper plugin. The support is on the m2e marketplace;
during the import of the Maven project, m2e will suggest to install it.
If it was imported previously, right-clicking on the error message will
suggest to install it, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Split code generation script into its own file

This keeps the POM easier to understand, while providing the actual code
generation logic within the project itself.
Remove trailing slashes from directory properties

Maven does not typically include the trailing slash in properties that
configure a directory path. So let's not do that here either.
Move the template files into src/main/templates/

Suggested by Curtis Rueden.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #2 from imagej/generated-sources

Add proof-of-concept compile time code generator