build.gradle: add javafx runtime dependency for all platforms
build.gradle: add javafx runtime dependency for all platforms

We have added the platform-specific javafx runtime dependencies, so the
addressbook is able to run locally.

However, the jar file generated on one OS cannot run on other OS. The
reason is that, after detecting the OS, javafx will try to load
platform-specific classes dynamically. Without corresponding javafx
dependencies for that other OS, the class loading process will fail.

Let's add the javafx runtime dependency for all platforms (MacOS,
Window, Linux) so the jar file generated on one OS is able to run in
other OS [1].

Following are some key facts that guarantee the solution to work:

  * Gradle places all dependency JARs on the classpath and the order is
    not defined by Gradle.

  * When shadowJar encounters duplicate classes with same package name
    and same class name, it picks the first one it sees. ShadowJar
    processes dependency JARs according to their order in classpath.

  * The platform-specific JARs for different OSs can define the same
    classes. (e.g. javafx-graphics-11-win.jar and
    javafx-graphics-11-linux.jar both contain
    javafx/scene/control/TreeView.class). However, via diffing the
    contents of the JARs, we have verified that if two JARs define the
    same class, the class files are also identical to each other. So, it
    doesn't matter if the TreeView.class from javafx-graphics-11-win.jar
    or the TreeView.class from javafx-graphics-11-linux.jar is loaded.
    As such, no matter what order Gradle places JavaFX's JARs on the
    classpath, we can run and distribute addressbook properly.

[1] https://stackoverflow.com/questions/52653836/maven-shade-javafx-runtime-components-are-missing/52654791#52654791

se-edu/addressbook-level3