Take the right button away from the cable tool only, not from every tool

The cable feature suppressed the quick command ring for every running
tool, so drawing a line or placing an element no longer got the ring a
right drag shows -- a far wider change than the feature needed, and one
nobody asked for (blocking review comment by ispyisail).

The condition is now the cable tool alone. Diagram::cableToolIsRunning()
says whether that is the running interface, and while it is, the right
click cancels the line being drawn and opens neither the ring nor a
context menu, neither QET's own nor the platform's -- which is the
behaviour the cable tool needs and the only part that was ever intended.

Everything else in mousePressEvent is upstream's original again: a
gesture is tracked whatever else is running, a running tool sets
m_gesture_over_tool so the press carries on to the scene and a drag ends
the tool before the ring appears, and only the click which neither draws
nor selects swallows the platform's menu. A structural diff of
mousePressEvent against the commit before the cable feature shows three
added conditions and nothing else.

sources/diagram.cpp

@@ -25,6 +25,7 @@
 #include "xml/terminalstripitemxml.h"
 #include "QPropertyUndoCommand/qpropertyundocommand.h"
 #include "diagramcontent.h"
+#include "diagramevent/diagrameventaddcable.h"
 #include "diagramevent/diagrameventinterface.h"
 #include "diagramposition.h"
 #include "factory/elementfactory.h"
@@ -902,6 +903,23 @@ bool Diagram::eventInterfaceIsRunning() const
 	return m_event_interface && m_event_interface->isRunning();
 }
 
+/**
+	@brief Diagram::cableToolIsRunning
+	@return true while the cable drawing tool is the running tool.
+
+	That tool is the one which takes the right mouse button away from
+	everything else: while it runs, a right click cancels the line being
+	drawn and must not open the quick command ring nor any context menu
+	with it. Every other tool lets go of the button again and is
+	surrounded by the ring the way it always was.
+*/
+bool Diagram::cableToolIsRunning() const
+{
+	return m_event_interface
+	       && m_event_interface->isRunning()
+	       && qobject_cast<DiagramEventAddCable *>(m_event_interface);
+}
+
 /**
 	@brief Diagram::conductorsAutonumName
 	@return the name of autonum to use.