Add ability to configure classpathEntries from gradle.

[PokeMMO]

This should allow building an alternative configuration other than the default runtimeClasspath. This can be used to build a more finely controlled classpath, including prebuilt jars while keeping the quality of life of the gradle plugin.

configurations { custom }

dependencies {
    custom "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-ios"
    custom "com.badlogicgames.gdx:gdx-freetype-platform:$gdxVersion:natives-ios"
    custom files('local_jar.jar')
}

robovm {
    configurationName custom
    skipDefaultClasspathEntries true
}

This PR is untested, but I believe it should compile/function as intended.

[dkimitsa]

hi, thank you for contribution.
Can't all this be done by Gradle directly?
The only specific thing is robovm.skipDefaultClasspathEntries but I doubt
it is needed by most of community (actually these folders were added by community to workaround all kind of issues).
Second moment -- how it is going to play with Idea plugin.