Don't require leading TAB in wxAcceleratorEntry::FromString().

Don't require leading TAB in wxAcceleratorEntry::FromString().

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