Use separate strings for stock labels with and without mnemonics.

Use separate strings for stock labels with and without mnemonics.

Obtaining the string without mnemonics by simply removing "&" characters from
the string containing mnemonics doesn't work for some languages, notably
Chinese where the convention is to use "Chinese Text (&M)" for the labels with
"M" being the ASCII mnemonic and just "Chinese Text" and not "Chinese Text (M)"
should be used if wxSTOCK_WITH_MNEMONIC flag is not specified.

Fix the fundamental problem by using separate strings for the two cases.
Translations still need to be updated to really correct the labels appearance.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68641 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775