Keyboard: properly handle keyboard layout height change

The japanese keyboard being taller than the others, when
switching to/from it from/to another layout:
- re-init InputDialog for proper sizing and positionning
- refresh the whole screen, to remove any trace of
  a previous taller keyboard

Also add calls to :free() here and there to free keyboard
keys' TextWidgets' XText C objects without waiting for GC.

frontend/ui/widget/inputtext.lua

@@ -406,6 +406,13 @@ function InputText:onCloseKeyboard()
     UIManager:close(self.keyboard)
 end
 
+function InputText:onCloseWidget()
+    if self.keyboard then
+        self.keyboard:free()
+    end
+    self:free()
+end
+
 function InputText:getTextHeight()
     return self.text_widget:getTextHeight()
 end