<AWT Dev> RFR: 8252015: [macos11] java.awt.TrayIcon requires updates for template images

[Peter Zhelezniakov]

[JDK-8252015: [macos11] java.awt.TrayIcon requires updates for template
images](https://bugs.openjdk.java.net/browse/JDK-8252015)

### Problem
According to Apple's human interface guidelines, developers should use template images for tray icons. This way icons
look good when desktop theme or wallpaper change. On the API level, this means setting the `NSImage::isTemplate` flag.
Currently there's no way in Java to set this flag. Even if one uses a template (black and transparent pixels only)
image, MacOS will treat it as regular image. As a result, the image will not be visible when using dark theme.

### Solution:
* If `sun.awt.enableTemplateImages` property is set, all tray icon images are treated as templates. It's the developers'
  responsibility to ensure their images are indeed templates. This property allows us to avoid new API and accidental
  behavior changes
* Value of this property is passed from Java into `NSImage::setTemplate`
* We need the tray icon image rendered by MacOS, so we use a button (rather than a view) to host the image

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Commit messages:
 - 8252015: [macos11] java.awt.TrayIcon requires updates for template images

Changes: https://git.openjdk.java.net/jdk/pull/481/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=481&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252015
  Stats: 214 lines in 3 files changed: 69 ins; 81 del; 64 mod
  Patch: https://git.openjdk.java.net/jdk/pull/481.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/481/head:pull/481

PR: https://git.openjdk.java.net/jdk/pull/481