Whole code base: various cleanups, removed SINGLE_SOURCE_COMPILE relicts

* cleaned up code and improved coding style
* removed old SINGLE_SOURCE_COMPILE macro relicts
* use QString::toUtf8() instead of QString::toAscii() wherever possible

(cherry picked from commit 0bb54199b0)

src/gui/plugin_browser.cpp

@@ -1,5 +1,3 @@
-#ifndef SINGLE_SOURCE_COMPILE
-
 /*
  * plugin_browser.cpp - implementation of the plugin-browser
  *
@@ -225,4 +223,3 @@ void pluginDescWidget::updateHeight( void )
 #include "moc_plugin_browser.cxx"
 
 
-#endif