Gradle maintenance

[dkimitsa]

this pr contains following changes:

1. added dependency to build task to force java compilation for every RoboVM task. Otherwise commands like launchIOSDevice will fail as java classes are not generated.

2. [cosmetic] added description to tasks

> gradle tasks
MobiVM tasks
------------
createIPA - Creates .ipa file. This is an alias for the robovmArchive task
launchConsole - Runs a console app
launchIOSDevice - Runs your iOS app on a connected iOS device.
launchIPadSimulator - Runs your iOS app in the iPad simulator
launchIPhoneSimulator - Runs your iOS app in the iPhone simulator
robovmArchive - Compiles a binary, archives it in a format suitable for distribution and saves it to build/robovm/
robovmInstall - Compiles a binary and installs it to build/robovm/

3. added logic not extracting same file on every launch and do clear cache when new files extracted just to avoid bug cases when incompatible data stays in cache, check post by @dthommes.
4. added support for launching debugger, similar as @dukescript did for 'maven plugin':

.. start with
> gradle --no-daemon -i -Probovm.debug=true -Probovm.debugPort=7778 -Probovm.arch=x86_64 "-Probovm.device.name=iPhone SE" launchIPhoneSimulator
.. attach with
> jdb -attach 7778