Switch to latest official SWT releases from Maven Central

[simonbasle]

SWT has started being officially distributed to Maven Central as of early 2017.
As a result, https://maven-eclipse.github.io/maven has not been updated in a while, and the build is currently outdated.

We could switch to latest SWT distribution using the swt platform, which allows us to declare a dependency to an OS-independent artifact (which in turn declares a dependency to ${osgi.platform}-dependent jar).

For compilation purposes, the osgi.plaftform must be substituted with a locally-relevant os-specific jar, like so:

	configurations.all {
		resolutionStrategy {
			dependencySubstitution {
				// The maven property ${osgi.platform} is not handled by Gradle
				// so we replace the dependency, using the osgi platform from the project settings
				substitute module('org.eclipse.platform:org.eclipse.swt.${osgi.platform}') with module("org.eclipse.platform:org.eclipse.swt.${getSwtPlatform()}:${swtVersionPlatform}")
			}
		}
	}


	dependencies {
		optional "org.eclipse.platform:org.eclipse.swt:${swtVersionPlatform}"
	}

Latest swtVersionPlatform is 3.112.0. Notice the official Maven Central groupId is org.eclipse.platform.

[simonbasle]

api-change: not really API per-se, but the dependency to SWT would change.

[simonbasle]

⚠️ The same type of change would be necessary in https://github.com/reactor/reactor-kotlin-extensions !

[simonbasle]

superseded by #273