Fix highlighting of commands including non-ASCII letters (fixes #67)

With the migration from QRegExp to QRegularExpression in 481eb4e,
unicode character classes can now be used in the syntax pattern
definitions.

res/resfiles/configuration/syntax-patterns.txt

@@ -42,7 +42,7 @@ darkgreen	N	\\(?:begin|end)\s*\{[^\}]*\}
 darkblue	N	\\usepackage\s*(?:\[[^\]]*\]\s*)?\{[^\}]*\}
 
 # control sequences
-blue		N	\\(?:[A-Za-z@]+|.)
+blue		N	\\(?:[\p{L}@]+|.)
 
 # comments
 red		Y	%.*
@@ -61,7 +61,7 @@ gray		N	[\-+/^_]
 darkgreen;B	N	\\(?:start|stop)[A-Za-z]+
 
 # control sequences
-blue;B		N	\\(?:[A-Za-z@]+|.)
+blue;B		N	\\(?:[\p{L}@]+|.)
 
 # comments
 red		Y	%.*
@@ -105,7 +105,7 @@ green		N	\\(?:begin|end)\{macrocode\}
 darkgreen	N	\\(?:begin|end)\s*\{[^}]*\}
 
 # control sequences
-blue		N	\\(?:[A-Za-z@:_]+|.)
+blue		N	\\(?:[\p{L}@:_]+|.)
 
 [Lua]