Implement HiDPI

@pmouawad ([Bug 59168](https://bz.apache.org/bugzilla//show_bug.cgi?id=59168&redirect=false)):
Following improvements made in https://github.com/apache/jmeter/issues/3674, this bug is created to take into account the following proposals from Benoit Wiart:

on macos loading @2x resources is supported since jdk8u20
- see https://bugs.openjdk.java.net/browse/JDK-8011059

The user should not choose a different size in the properties to get hdpi support.

hdpi image is coming to jdk9
- https://bugs.openjdk.java.net/browse/JDK-8046010
- http://openjdk.java.net/jeps/251

- do not mix image size and hdpi
- use the "standard" of @2x, @3x convention for the images instead of the <SIZE> hack.
- use a single code path to load icons : it will allow caller to know nothing about hdpi
for example the caller want to load /status/myimage.png

if in hdpi the icon loader will try to load /status/myimage@2x.png if not found it will fall back to /status/myimage.png

it will allow third party plugins to progressively provide @2x resources

Some help, it's old and only cover macos but ...
http://bulenkov.com/2013/06/23/retina-support-in-oracle-jdk-1-7/
http://bulenkov.com/iconloader/

OS: All

Depends on:
* https://github.com/apache/jmeter/issues/3674