VirtualKeyboard: Revamp visibility handling (#10852)

Move as much of the state tracking as possible inside VirtualKeyboard itself.
InputDialog unfortunately needs an internal tracking of this state because it needs to know about it *before* the VK is shown, so we have to keep a bit of duplication in there, although we do try much harder to keep everything in sync (at least at function call edges), and to keep the damage contained to, essentially, the toggle button's handler.

(Followup to #10803 & #10850)

frontend/ui/widget/multiinputdialog.lua

@@ -218,7 +218,9 @@ end
 function MultiInputDialog:onSwitchFocus(inputbox)
     -- unfocus current inputbox
     self._input_widget:unfocus()
-    self._input_widget:onCloseKeyboard()
+    -- and close its existing keyboard (via InputDialog's thin wrapper around _input_widget's own method)
+    self:onCloseKeyboard()
+
     UIManager:setDirty(nil, function()
         return "ui", self.dialog_frame.dimen
     end)
@@ -226,7 +228,9 @@ function MultiInputDialog:onSwitchFocus(inputbox)
     -- focus new inputbox
     self._input_widget = inputbox
     self._input_widget:focus()
-    self._input_widget:onShowKeyboard()
+
+    -- Make sure we have a (new) visible keyboard
+    self:onShowKeyboard()
 end
 
 return MultiInputDialog