Metamenus

- Fixed ArtProvider hardcoded to 16x16 icons.

- The "bmp" kwarg, when receives a string, will now also try to find a gtk
stock bitmap if it fails to find a bitmap on the art provider. E. g.,
a "cdrom" value will fetch the bitmap for "gtk-cdrom". Full list here:
<https://wxpython.org/Phoenix/docs/html/stock_items.html>

- MenuAfter/MenuBefore are now command events.

- Added item=attr_name to event on OnMB_after

- Now messages warning about methods not found on parent are not shown
by default. To enable them again, change the _verbose constant to True.

- Tries to use unidecode on _clean() to remove accents.

- GetItemState should now return False instead of raising an error if
a tested item is not checkable. Testing on wxPython 4.1.0.

- Sintax adapted to python3 and wxPython Phoenix
(e. g. Phoenix doesn't resolve wx.ITEM_CHECK as an int anymore; now it's
a wx._core.ItemKind)

- Fix in place for Phoenix breaking the tree parsing algorithm
as per https://github.com/wxWidgets/Phoenix/issues/1648
(I'm using 4.0.7p2 right now and the fix seems to be holding.)

- Added "bmp" as kwarg and alias to "bmpChecked" (SetBitmap), but "bmp"
also accepts strings: the value passed then must be the string
representation of a wx.ArtProvider id minus the 'ART_' prefix
(e. g. "CLOSE" [or "close" -- it will be uppercase'd] if
you want the art provider bitmap for wx.ART_CLOSE).

- Removed xaccel; it was only needed for numpad accelerators but
Phoenix does them!

- Renamed MenuBarEx's GetMenuState and SetMenuState methods to GetItemState
and SetItemState for consistency.

- Added item=attr_name to event on OnM_after

- Removed evt.Skip() from the end of OnMB_ to avoid triggering the
event twice. Used to work with previous versions of wx though.

- Changed _clean() so that it doesn't remove accentuated chars anymore;
instead, it will try to 'remove' their accents.

- Some minor fixes.