Rewrite class UTF8 to contain rather than extend std::string storage.

This forces the compiler class specific features rather than borrowing
from the base class's std::string.  In some cases prior to this,
wxString( std::string ) was being called rather than UTF8::operator
wxString() leading to garbled wxStrings.

Added function UTF8::wx_str() which is of great convenience also.

Implicit conversions still work as before, and hopefully more reliably.

common/fp_lib_table.cpp

@@ -286,7 +286,7 @@ MODULE* FP_LIB_TABLE::FootprintLoad( const wxString& aNickname, const wxString&
         LIB_ID& fpid = (LIB_ID&) ret->GetFPID();
 
         // Catch any misbehaving plugin, which should be setting internal footprint name properly:
-        wxASSERT( aFootprintName == (wxString) fpid.GetLibItemName() );
+        wxASSERT( aFootprintName == fpid.GetLibItemName().wx_str() );
 
         // and clearing nickname
         wxASSERT( !fpid.GetLibNickname().size() );