Support symbol strings with '_' in place of '+'

This fixes a segfault that occurs for the following reason:

 - In XKeyboard::initializeXkb(), the XkbSymbolParser::parse() is called
   with three arguments - string to process (symName),
   vector to push symbol names to (_symbolNames)
   and vector to push variant names to (_variantNames).

 - In XkbSymbolParser::parse(), it looks for char '+' in symName string,
   but Xkb may use '_' instead when creating that string. If the char
   is not found, parse() doesn't append anything to vectors and they
   remain empty.

 - Later, XKeyboard::initializeXkb() tries to access nonexistent data in
   _symbolNames, which causes a segmentation fault (on lines 162-163).

XKeyboard.cpp

@@ -327,7 +327,7 @@ void XkbSymbolParser::parse(const std::string& symbols, StringVector& symbolList
     
     for (size_t i = 0; i < symbols.size(); i++) {
         char ch = symbols[i];
-        if (ch == '+') {
+        if (ch == '+' || ch == '_') {
             if (inSymbol) {
                 if (isXkbLayoutSymbol(curSymbol)) {
                     symbolList.push_back(curSymbol);