qt: support other QPA platforms

Platform::Invalid was renamed to Platform::Other when platform
identification moved to SalInstance, but the Qt VCL plugin retained
the assertions and aborts used for the former invalid sentinel.

Allow unrecognised Qt platform plugins to use the generic native-window
path. This lets third-party QPA plugins operate without requiring an
explicit LibreOffice platform classification.

Tested with the MWS (Mercurius Window System) Qt 5 QPA plugin by
starting LibreOffice Writer. Also exercised with Impress editing an image-heavy presentation.

Change-Id: Idb0be3d7d0b5508565d6beb17729f768e552bb85
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/211434
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/qt5/QtFrame.cxx

@@ -221,10 +221,8 @@ QWindow* QtFrame::windowHandle() const
         case Platform::Windows:
         case Platform::Xcb:
         case Platform::Mac:
-            pChild->setAttribute(Qt::WA_NativeWindow);
-            break;
         case Platform::Other:
-            std::abort();
+            pChild->setAttribute(Qt::WA_NativeWindow);
             break;
     }
     return pChild->windowHandle();

vcl/qt5/QtInstance.cxx

@@ -652,7 +652,6 @@ Platform QtInstance::GetPlatform() const
     if (sPlatformName == u"cocoa")
         return Platform::Mac;
 
-    assert(false && "Unsupported qt VCL platform");
     return Platform::Other;
 }