[Gtk4] Fix Shell.computeSize to take header bar into account

[akurtakov]

HeaderBar is part of the regular window size in Gtk 4 on Wayland BUT gtk_window_get_headerbar returns 0 if it is not set by the application but drawn internally by Gtk itself and thus it's size can not be queried. Add a "default" GtkHeaderBar explicitly for that and take its size into account when computing Shell size.

[akurtakov]

This fixes initial size of Eclipse main window and dialogs, resizing is still broken. Fixes github.com//issues/2826

[github-actions]

Test Results (linux)

   83 files   -  5     83 suites   - 5   10m 24s ⏱️ - 4m 10s
4 524 tests  - 33  4 311 ✅  - 26  212 💤  - 8  1 ❌ +1 
  175 runs   - 36    174 ✅  - 34    1 💤  - 2  0 ❌ ±0 

For more details on these failures, see this check.

Results for commit 124d5e5. ± Comparison against base commit d4f11be.

This pull request removes 33 tests.

org.eclipse.swt.tests.gtk.snippets.Bug336238_ShellSetBoundFailTest ‑ testSetBounds
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser ‑ test_ClearAllSessionCookies
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser ‑ test_OpenWindowListener_openHasValidEventDetails
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser ‑ test_OpenWindowListener_open_ChildPopup
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser ‑ test_OpenWindow_Progress_Listener_ValidateEventOrder
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser ‑ test_StatusTextListener_hoverMouseOverLink
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser ‑ test_VisibilityWindowListener_eventSize
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser ‑ test_VisibilityWindowListener_multiple_shells
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser ‑ test_get_set_Cookies
org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser_IE ‑ test_ClearAllSessionCookies
…

♻️ This comment has been updated with latest results.