FromString() should parse string returned by ToString() successfully but this
wasn't the case because the accelerator parsing functions always insisted on
having a TAB in the string.
Fix this, document the string format and add a unit test checking for the
correct behaviour.
Closes#12745.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66308 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
