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/ui/inditextpropertieswidget.cpp

@@ -20,6 +20,7 @@
 #include "../QPropertyUndoCommand/qpropertyundocommand.h"
 #include "../diagram.h"
 #include "../diagramcommands.h"
+#include "../lastusedstyle.h"
 #include "../qetgraphicsitem/independenttextitem.h"
 #include "../ui_inditextpropertieswidget.h"
 
@@ -455,6 +456,7 @@ void IndiTextPropertiesWidget::on_m_font_pb_clicked()
 		m_font_is_selected = true;
 		ui->m_font_pb->setText(font.family());
 		ui->m_size_sb->setValue(font.pointSize());
+		LastUsedStyle::setTextFont(m_selected_font);
 		apply();
 	} else {
 		ui->m_font_pb->setText(tr("Police"));