[Draw2D] Align Layer with Draw2D implementation

[ptziegler]

The layer name should not be stored as a local variable in the Layer class. Instead it should be used as constraint when adding the figure to the root figure.

Note that the root figure still keeps track of all registered layers by name. This will be changed in the future, where the layers are added to a LayeredPane instead. It is also not yet possible to remove this forked class, as it is used in methods which are API. Deprecation will be handled in a follow-up PR.

[github-actions]

Test Results

     4 files  ±0       4 suites  ±0   2h 25m 25s ⏱️ + 2m 6s
 7 019 tests  - 3   6 989 ✅  - 3  30 💤 ±0  0 ❌ ±0 
23 680 runs   - 4  23 584 ✅  - 4  96 💤 ±0  0 ❌ ±0 

Results for commit 455428c. ± Comparison against base commit 3dd1906.

This pull request removes 3 tests.

org.eclipse.wb.tests.draw2d.LayerTest ‑ test_bounds
org.eclipse.wb.tests.draw2d.LayerTest ‑ test_name
org.eclipse.wb.tests.draw2d.LayerTest ‑ test_opaque

[ptziegler]

The Jenkins test failure is similar to #1645. I'll prepare a PR for that separately.