src: clazy compiler warnings

get rid of most clazy warnings (https://github.com/KDE/clazy)
* range loop use refs
* qcolor ctor with ints
* missing Q_OBJECT macro
* use .constfirst instead of .first

enabled ccache for unix systems for fasterr recompiling
* if ccache is in path we use it now

src/lib_gui/qt/element/QtDirectoryListBox.cpp

@@ -228,7 +228,6 @@ bool QtDirectoryListBox::event(QEvent* event)
 
 void QtDirectoryListBox::dropEvent(QDropEvent *event)
 {
-	QFileInfo fileInfo;
 	foreach(QUrl url, event->mimeData()->urls())
 	{
 		QtListItemWidget* widget = addListBoxItem();