Don't pass strings not containing accelerators to ParseAccel().

Don't pass strings not containing accelerators to ParseAccel().

Check for the presence of accelerator part in the string passed to
wxAcceleratorEntry::Create() and don't call ParseAccel() at all if it's not
there. This avoids the spurious warnings about unrecognized accelerators when
creating menu items that don't have any accelerators at all.

Also update wxAcceleratorEntry::FromString() documentation to mention that
the new code should pass just the accelerator to this function and that it
only accepts full menu item labels for compatibility.

Closes #12770.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66379 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775