UnRedo: Explicitly clear the commands history when a file is reloaded

Normally the commands were cleared anyway, but maybe not always with a non-workspace file.
Disable the LabelCurrentState menuitem when there's no genuine editor
UnRedo: Prevent a stale native-toolbar dropdown menu showing though there's no active editor

The native toolbar dropdown tool menu displays its menu if the wxEVT_TOOL_DROPDOWN is skipped. However it still owns the last-used menu, so if a valid menu isn't attached before the Skip(), the stale previous menu is still shown.
UnRedo: When opening the Edit menu, remove any stale LabelledStates submenu

This was already done when the active tab contained a 'real' editor. Do it too when it's not.