Add Maven launch-config to build native binaries of running platform

[HannesWell]

In the SWT maven build local building of the native binaries can be activated by setting the system property native to ${ws}.${os}.${arch}.
Consequently the full command for a complete Maven build of SWT that also builds the natives is (with the SWT-repo root as working directory):
mvn clean verify -Dnative=${ws}.${os}.${arch}

This full build takes a few minutes and if one skips the test execution, by appending -DskipTests, the complete build terminates earlier.

The fastest way to build the native binaries for a platform with Maven is to only run the execution of the maven-antrun-plugin that contains the local natives build. On the CLI this would be (with the SWT-repo root as working directory):
mvn clean antrun:run@build-native-binaries -f binaries/org.eclipse.swt.${ws}.${os}.${arch} -Dnative=${ws}.${os}.${arch}

This PR adds a Maven launch configuration (requires Eclipse m2e) to the org.eclipse.swt project to perform such native binaries only build for the currently running platform. The launch-config uses the latter Maven command and infers the ${ws}, ${os} and ${arch} placeholder using the launch variables ${target.ws}, ${target.os} and ${target.arch}. With that one can just build the natives without further configuration after the necessary prerequisites are installed as described in the corresponding articles in https://www.eclipse.org/swt/docs.php.

[github-actions]

Test Results

   299 files  ±0     299 suites  ±0   5m 53s ⏱️ -18s
 4 098 tests ±0   4 090 ✅ ±0   8 💤 ±0  0 ❌ ±0 
12 206 runs  ±0  12 133 ✅ ±0  73 💤 ±0  0 ❌ ±0 

Results for commit 2d3eef1. ± Comparison against base commit 5c9180b.

[lathapatil]

For local testing what are the replacement commands or steps using maven build
6th step here may be outdated by now