Editor: Adjust valid sippet trigger

Change-Id: I67ac9cf2b853e077f3b073100788530529efad2d
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/texteditor/snippets/snippetssettingspage.cpp

@@ -258,7 +258,7 @@ bool SnippetsTableModel::isValidTrigger(const QString &s)
     if (s.isEmpty())
         return false;
     for (int i = 0; i < s.length(); ++i)
-        if (!s.at(i).isLetter())
+        if (!s.at(i).isLetter() && s.at(i) != QLatin1Char('_') && (!s.at(i).isDigit() || i == 0))
             return false;
     return true;
 }