added menuitem "Import from clipboard"
Merge pull request #16 from tessus/menu-import-clipboard

Adds a special menu item to the Key menu to directly import keys found in the clipboard.
[FIX] Remove undo/redo menu items until they work reliably [#309 state:fixed]

- At the moment redo and undo don't always work reliably, so they should not be available
  at all.