Untangle include forrest

Every change in connectionwindow.h led to a heavy project
compile, as the header was included in other headers.

Avoid this by using forward declarations and only include
connectionwindow.h in the C++ files that actually use it.

mainwindow.cpp

@@ -5,6 +5,7 @@
 #include <QFileDialog>
 #include <QtSerialPort/QSerialPortInfo>
 #include "connections/canconmanager.h"
+#include "connections/connectionwindow.h"
 #include "utility.h"
 
 /*