weld: Move weld::Dialog to own header/source

Move the class out of weld.hxx into its own header and source
file, in order to make it easier to keep an overview and to avoid
having to recompile ~all UI code when the class is
modified.

Change-Id: Ic47dac553fc065c5c9cddbf31e33012314dbe43d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/199073
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

test/source/screenshot_test.cxx

@@ -22,6 +22,7 @@
 #include <vcl/svapp.hxx>
 #include <vcl/virdev.hxx>
 #include <vcl/weld/Builder.hxx>
+#include <vcl/weld/Dialog.hxx>
 #include <vcl/weld/Notebook.hxx>
 #include <vcl/weld/weld.hxx>
 #include <tools/stream.hxx>