Force the view switcher dialog to always be centered on the window

The dialog isn't actually movable with the mouse, so all the placement
logic inside DIALOG_SHIM is extraneous and actually causes the switcher
to appear on the wrong monitor if pcbnew was moved across monitors after
it was opened.

Fixes https://gitlab.com/kicad/code/kicad/issues/5789

common/dialogs/eda_view_switcher.cpp

@@ -70,8 +70,13 @@ bool EDA_VIEW_SWITCHER::Show( bool aShow )
     bool ret = DIALOG_SHIM::Show( aShow );
 
     if( aShow )
+    {
         m_receivingEvents = true;
 
+        // Force the dialog to always be centered over the window
+        Centre();
+    }
+
     return ret;
 }