[GTK] Replacing usages of Display#getDPI from gtk

[ShahzaibIbrahim]

Having the scale factor being based on the screen DPI leads to unexpected result e.g. Image too big/small. Having a screen dpi independent factor leads to consistent results.

[github-actions]

Test Results (linux)

   85 files   -  3     85 suites   - 3   10m 31s ⏱️ - 3m 42s
4 523 tests  - 33  4 311 ✅  - 25  212 💤  - 8  0 ❌ ±0 
  193 runs   - 18    191 ✅  - 17    2 💤  - 1  0 ❌ ±0 

Results for commit 68506f4. ± Comparison against base commit 833d748.

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
…

[jonahgraham]

The GTK3 tests hung after 360m - I don't think that is because of your change and I have raised #2876 to track that