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

tests/test_test_gui.dsp

@@ -235,6 +235,10 @@ LINK32=link.exe
 # PROP Default_Filter ""
 # Begin Source File
 
+SOURCE=.\menu\accelentry.cpp
+# End Source File
+# Begin Source File
+
 SOURCE=.\asserthelper.cpp
 # End Source File
 # Begin Source File
@@ -337,10 +341,10 @@ SOURCE=.\controls\headerctrltest.cpp
 # End Source File
 # Begin Source File
 
-SOURCE=.\controls\htmllboxtest.cpp
-# End Source File
-# Begin Source File
-
+SOURCE=.\controls\htmllboxtest.cpp
+# End Source File
+# Begin Source File
+
 SOURCE=.\html\htmlwindow.cpp
 # End Source File
 # Begin Source File