Add contract editor

[Felix-Schmid]

The Xtext based editor is now used for editing contracts. Also, instead of storing contracts in the comment of the SubApp, it is now stored as an attribute and displayed with a new EditPart.

[github-actions]

Test Results

   109 files   - 4     109 suites   - 4   49s ⏱️ ±0s
29 180 tests  - 5  29 180 ✅  - 5  0 💤 ±0  0 ❌ ±0 
29 181 runs   - 5  29 181 ✅  - 5  0 💤 ±0  0 ❌ ±0 

Results for commit 86ec87f. ± Comparison against base commit eee5351.

This pull request removes 6 and adds 1 tests. Note that renamed tests count towards both.

org.eclipse.fordiac.ide.test.model.contracts.AssumptionTest ‑ test
org.eclipse.fordiac.ide.test.model.contracts.ContractTest ‑ testNotValid
org.eclipse.fordiac.ide.test.model.contracts.ContractTest ‑ testValid
org.eclipse.fordiac.ide.test.model.contracts.GuaranteeTest ‑ test
org.eclipse.fordiac.ide.test.model.contracts.SimplifyAssumptionTest ‑ test
org.eclipse.fordiac.ide.test.model.contracts.SimplifyGuaranteeTest ‑ test

org.eclipse.fordiac.ide.test.model.contracts.ContractTest ‑ dummyTest

♻️ This comment has been updated with latest results.