Added 'current line number' to color theme settings

Recently it was made to highlight black or write, which isn't nice for
everybody. Now it is dark gray by default, and can be customized. Bold
and italic are also supported.

src/plugins/texteditor/texteditorconstants.h

@@ -72,6 +72,7 @@ const char * const C_SEARCH_RESULT       = "SearchResult";
 const char * const C_SEARCH_SCOPE        = "SearchScope";
 const char * const C_PARENTHESES         = "Parentheses";
 const char * const C_CURRENT_LINE        = "CurrentLine";
+const char * const C_CURRENT_LINE_NUMBER = "CurrentLineNumber";
 
 const char * const C_NUMBER              = "Number";
 const char * const C_STRING              = "String";