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/qetgraphicsitem/independenttextitem.cpp

@@ -19,6 +19,7 @@
 
 #include "../diagram.h"
 #include "../diagramcommands.h"
+#include "../lastusedstyle.h"
 #include "../qet.h"
 #include "../qetapp.h"
 #include "../utils/qetutils.h"
@@ -33,7 +34,10 @@
 IndependentTextItem::IndependentTextItem() :
 	DiagramTextItem(nullptr)
 {
-	setFont(QETApp::indiTextsItemFont());
+		//Start from the font last applied to a text item this session,
+		//falling back to the app-wide Preferences default otherwise.
+	setFont(LastUsedStyle::hasTextFont() ? LastUsedStyle::textFont()
+					      : QETApp::indiTextsItemFont());
 	QSettings settings;
 	setRotation(settings.value("diagrameditor/independent_text_rotation", 0).toInt());
 }