flatmenu memory usage improvements

[arjones6]

Makes FlatMenuItem check mark and radio button bitmaps per class variables instead of per instance variables so that individual bitmaps aren't created for each menu item. Also cleans up child menu items if a parent menu is explicitly destroyed.

Fixes #2372

[RobinD42]

This pull request has been mentioned on Discuss wxPython. There might be relevant details there:

https://discuss.wxpython.org/t/advice-on-approach-to-menu-editor/36994/9

[komoto48g]

@swt2c, the merged commit has TabError.
@emersonrp, please revert and make a new PR, if you have time.
In addition, Destroy is expected to return a bool and raise a TypeError if it does not.
For example, it is preferable to change it to:

def Destroy(self, *args, **kwargs):
    try:
        self.Clear()
    finally:
        return super().Destroy(*args, **kwargs)

[swt2c]

@komoto48g fixed.