logic: used wstring in FilePath constructor

* used wstring in FilePath constructor where easily possible
* added FilePath concatenate method that accepts string parameter
* implemented using only wstring in MessageStatus
* used wstring in config

bin/test/data/ConfigManagerTestSuite/test_data.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<config>
+    <path>
+        <to>
+            <special_character>ü</special_character>
+        </to>
+    </path>
+</config>

bin/test/data/SettingsTestSuite/settings.xml

@@ -4,6 +4,7 @@
 	<Int>42</Int>
 	<Float>3.1416</Float>
 	<String>Hello World!</String>
+	<WString>Hello World!</WString>
 
 	<source_groups>
         <source_group_77438437-02db-4207-8bbf-75c627d53b89>