Remember last-used shape/text style for new items this session

Drawing tools on the diagram canvas always started new shapes and free
text from a fixed hardcoded default (Qt's plain QPen()/QBrush(), and
the static Preferences font) -- changing a shape's color or a text's
font had zero effect on what the next new item of that type got, even
within the same editing session.

Add LastUsedStyle: a small in-memory, session-scoped static helper
(no QSettings, no persistence across restarts -- this is a live "what
did I just use" value, not a new app-wide default). Write side hooks
capture the value right where the properties editors already apply a
change (ShapeGraphicsItemPropertiesWidget::associatedUndo(), both the
live-edit dock path and the modal editProperty() dialog path; and
IndiTextPropertiesWidget::on_m_font_pb_clicked() right after the font
dialog returns). Read side hooks apply the stored value, if any, to a
newly created item: DiagramEventAddShape::mousePressEvent for shapes,
IndependentTextItem's constructor for free text (falling back to the
existing QETApp::indiTextsItemFont() Preferences default otherwise).

Doesn't touch the element/symbol editor's own drawing tools (a
separate subsystem) or add last-used text color (no color control
exists in the text properties UI to originate it from yet).

sources/diagramevent/diagrameventaddshape.cpp

@@ -18,6 +18,7 @@
 #include "diagrameventaddshape.h"
 
 #include "../diagram.h"
+#include "../lastusedstyle.h"
 #include "../undocommand/addgraphicsobjectcommand.h"
 
 /**
@@ -77,6 +78,14 @@ void DiagramEventAddShape::mousePressEvent(QGraphicsSceneMouseEvent *event)
 		if (!m_shape_item)
 		{
 			m_shape_item = new QetShapeItem(pos, pos, m_shape_type);
+				//Start from whatever pen/brush was last applied this
+				//session, rather than always the hardcoded default.
+			if (LastUsedStyle::hasShapePen()) {
+				m_shape_item->setPen(LastUsedStyle::shapePen());
+			}
+			if (LastUsedStyle::hasShapeBrush()) {
+				m_shape_item->setBrush(LastUsedStyle::shapeBrush());
+			}
 			m_diagram->addItem (m_shape_item);
 			event->setAccepted(true);
 			return;