weld: Move weld::Label to own header

Move the class out of weld.hxx into its own header, 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: Ide86f35543468a88288a068cc9f89eaf381ac52f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/205763
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

basic/source/runtime/inputbox.cxx

@@ -27,6 +27,7 @@
 #include <vcl/weld/Dialog.hxx>
 #include <vcl/weld/DialogController.hxx>
 #include <vcl/weld/Entry.hxx>
+#include <vcl/weld/Label.hxx>
 #include <vcl/weld/weld.hxx>
 #include <rtlproto.hxx>
 #include <memory>

basic/source/runtime/iosys.cxx

@@ -24,6 +24,7 @@
 #include <vcl/weld/Dialog.hxx>
 #include <vcl/weld/DialogController.hxx>
 #include <vcl/weld/Entry.hxx>
+#include <vcl/weld/Label.hxx>
 #include <vcl/weld/MessageDialog.hxx>
 #include <vcl/weld/weld.hxx>
 #include <osl/diagnose.h>