Add accelerator shortcuts to menu items (#22)

Add accelerator shortcuts to menu items (#22)

* Add accelerator shortcuts to menu items

* Rename "Open" menu item to "Open...".
* Rename "SaveAs" menu item to "Save As...".
* Change some menu item mnemonics:
  * "New Output _Channel" (different from "_Open...")
  * "_Edit Input Channel"
  * "E_dit Output Channel"
  * "_Remove Input Channel"
  * "Re_move Ouput Channel"
  * "_Clear" (added)
* Disable "Edit/Remove Input Channel" sub-menu entries initially and
  when no input channels are left.
* Same for output channels.

* Change mnemonics to what is described in previous commit message

Signed-off-by: Christopher Arndt <chris@chrisarndt.de>