Don't use ternary operator ?: with wxString and literal strings.

This results in warnings about converting string literal to (non const) C
string.

Also remove the now unneeded calls to c_str() inside wxString::Printf().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73520 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775