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).

cmake/qet_compilation_vars.cmake

@@ -205,6 +205,8 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/exportpropertieswidget.h
   ${QET_DIR}/sources/genericpanel.cpp
   ${QET_DIR}/sources/genericpanel.h
+  ${QET_DIR}/sources/lastusedstyle.cpp
+  ${QET_DIR}/sources/lastusedstyle.h
   ${QET_DIR}/sources/machine_info.cpp
   ${QET_DIR}/sources/machine_info.h
   ${QET_DIR}/sources/main.cpp