Add an optional label to pictures (#349)

A picture can now carry a caption, set from its properties panel
("Libellé"). It is drawn centred under the picture at the folio's
normal text size whatever the picture's scale, turns with it, and
moves, copies and prints with it because the picture itself paints it.
Clicking the caption selects the picture.

Saved as a "label" attribute on <image>, written only when non-empty:
a project without labels saves byte-for-byte as before, and older
versions open a labelled project and simply ignore the caption.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G2d2Zi8BfrYRPX88zhaoFG

sources/ui/imagepropertieswidget.ui

@@ -152,14 +152,28 @@
      </property>
     </widget>
    </item>
-   <item row="6" column="0" colspan="2">
+   <item row="6" column="0">
+    <widget class="QLabel" name="label_label">
+     <property name="text">
+      <string>Libellé</string>
+     </property>
+    </widget>
+   </item>
+   <item row="6" column="1" colspan="2">
+    <widget class="QLineEdit" name="m_label_le">
+     <property name="toolTip">
+      <string>Texte affiché sous l'image. Il suit l'image quand elle est déplacée, copiée ou tournée.</string>
+     </property>
+    </widget>
+   </item>
+   <item row="7" column="0" colspan="2">
     <widget class="QCheckBox" name="m_lock_pos_cb">
      <property name="text">
       <string>Verrouiller la position</string>
      </property>
     </widget>
    </item>
-   <item row="7" column="0">
+   <item row="8" column="0">
     <spacer name="verticalSpacer">
      <property name="orientation">
       <enum>Qt::Vertical</enum>