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.

connections/canconnectionmodel.h

@@ -5,9 +5,8 @@
 #include <QAbstractTableModel>
 
 #include "canbus.h"
-#include "connections/canconnection.h"
-#include "connectionwindow.h"
 
+class CANConnection;
 
 class CANConnectionModel : public QAbstractTableModel
 {

mainwindow.cpp

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

mainwindow.h

@@ -22,7 +22,6 @@
 #include "mainsettingsdialog.h"
 #include "firmwareuploaderwindow.h"
 #include "re/discretestatewindow.h"
-#include "connections/connectionwindow.h"
 #include "scriptingwindow.h"
 #include "re/rangestatewindow.h"
 #include "dbc/dbcloadsavewindow.h"
@@ -33,6 +32,7 @@
 #include "motorcontrollerconfigwindow.h"
 #include "signalviewerwindow.h"
 
+class CANConnection;
 class ConnectionWindow;
 class ISOTP_InterpreterWindow;
 class ScriptingWindow;