Fix TextInputCutCopyPaste widget

[misl6]

The changes introduced in PR #7882 have broken the usage of the TextInputCutCopyPaste widget in a TextInput.

Minimal runnable example that reproduces the issue fixed in this PR:

TextInput:
    use_bubble: True
    text: "This is a test"

Error log:

Traceback (most recent call last):
   File "/Users/mirko/Documents/projects/kivy/examples/demo/kivycatalog/main.py", line 192, in <module>
     KivyCatalogApp().run()
   File "/Users/mirko/Documents/projects/kivy/kivy/app.py", line 955, in run
     runTouchApp()
   File "/Users/mirko/Documents/projects/kivy/kivy/base.py", line 574, in runTouchApp
     EventLoop.mainloop()
   File "/Users/mirko/Documents/projects/kivy/kivy/base.py", line 339, in mainloop
     self.idle()
   File "/Users/mirko/Documents/projects/kivy/kivy/base.py", line 379, in idle
     Clock.tick()
   File "/Users/mirko/Documents/projects/kivy/kivy/clock.py", line 733, in tick
     self.post_idle(ts, self.idle())
   File "/Users/mirko/Documents/projects/kivy/kivy/clock.py", line 776, in post_idle
     self._process_events()
   File "kivy/_clock.pyx", line 620, in kivy._clock.CyClockBase._process_events
     cpdef _process_events(self):
   File "kivy/_clock.pyx", line 653, in kivy._clock.CyClockBase._process_events
     self.handle_exception(e)
   File "kivy/_clock.pyx", line 649, in kivy._clock.CyClockBase._process_events
     event.tick(self._last_tick)
   File "kivy/_clock.pyx", line 218, in kivy._clock.ClockEvent.tick
     ret = callback(self._dt)
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/textinput.py", line 1488, in long_touch
     self._show_cut_copy_paste(
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/textinput.py", line 1895, in _show_cut_copy_paste
     self._bubble = bubble = TextInputCutCopyPaste(textinput=self)
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/textinput.py", line 306, in __init__
     super().__init__(**kwargs)
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/bubble.py", line 406, in __init__
     super().__init__(**kwargs)
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/boxlayout.py", line 145, in __init__
     super(BoxLayout, self).__init__(**kwargs)
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/layout.py", line 76, in __init__
     super(Layout, self).__init__(**kwargs)
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/widget.py", line 366, in __init__
     self.apply_class_lang_rules(
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/widget.py", line 470, in apply_class_lang_rules
     Builder.apply(
   File "/Users/mirko/Documents/projects/kivy/kivy/lang/builder.py", line 540, in apply
     self._apply_rule(
   File "/Users/mirko/Documents/projects/kivy/kivy/lang/builder.py", line 659, in _apply_rule
     widget.add_widget(child)
   File "/Users/mirko/Documents/projects/kivy/kivy/uix/bubble.py", line 416, in add_widget
     raise BubbleException(
 kivy.uix.bubble.BubbleException: Bubble can only contain a single Widget or Layout

Additionally, a layout issue was also introduced:

Expected:

After the changes introduced in #7882:

⚠️ After PR #7882, the bubble arrow may need some fixes to properly show on retina displays. It may be necessary to change the resource in order to avoid pixelation. (I have a retina-display-related WIP, so it will be managed separately)

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

[NomadDemon]

hmm do we have many places with hardcoded strings in kivy?