RFR: 8298500: Create test to initially show stage with various attributes (iconified, maximized, full screen)

[Lukasz Kostyra]

PR adds tests mentioned in the title - a new `AttributesTest` class is added testing iconification, maximization and full-screen-ification of a Stage.

All variants are tested with decorated stage style.

Iconification is tested via overlaying two stages on top of one another, and then iconifying the top one - this is similar to already existing `IconifyTest.java` but it tests just the iconfication process and nothing more.

Maximization and FullScreen are both tested by creating two stages _not_ overlapping each other. After maximization/fullscreen top stage (being always on top as well) should cover the bottom stage. Moreover, FullScreen and Maximize are differentiated by checking if window decoration exists - maximized Stage will have its decoration taking space on top of the screen, whereas FullScreen one will not.

**NOTE:** on macOS I had issues with `getColor()` returning a valid color when called a second time. This only happened on macOS and with FullScreen test (others worked fine). Unfortunately I couldn't figure out why it returned (0, 0, 0, 255) or (255, 255, 255, 255). To mitigate that I moved color checks into separate `runAndWait()`-s with a small sleep between them, which seemed to help `getColor()` return proper values.

Verified to work on Windows 11, macOS and Linux.

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Commit messages:
 - Add Stage Attributes robot test

Changes: https://git.openjdk.org/jfx/pull/1240/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1240&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298500
  Stats: 187 lines in 1 file changed: 187 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1240.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1240/head:pull/1240

PR: https://git.openjdk.org/jfx/pull/1240