Add user-defined custom properties on elements

ElementInfoWidget's fixed ~40 predefined ELMT_* keys had no way for a
user to add a genuinely new element-info key, even though DiagramContext
already stores/round-trips arbitrary keys generically via toXml()/fromXml().

Adds an "Ajouter une propriété personnalisée" button that appends a
CustomElementInfoPartWidget row (both key and value user-editable,
unlike the fixed ElementInfoPartWidget rows bound to one predefined
key). The typed key is validated live against the existing
DiagramContext::isKeyAcceptable() and flagged with a red border when
it doesn't match, instead of silently dropping it. Any key already
present on the element that isn't one of the predefined/special keys
is re-displayed as a custom row on next selection.

Implements the scope proposed in discussion #611.

cmake/qet_compilation_vars.cmake

@@ -683,6 +683,8 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/ui/dynamicelementtextitemeditor.h
   ${QET_DIR}/sources/ui/dynamicelementtextmodel.cpp
   ${QET_DIR}/sources/ui/dynamicelementtextmodel.h
+  ${QET_DIR}/sources/ui/customelementinfopartwidget.cpp
+  ${QET_DIR}/sources/ui/customelementinfopartwidget.h
   ${QET_DIR}/sources/ui/elementinfopartwidget.cpp
   ${QET_DIR}/sources/ui/elementinfopartwidget.h
   ${QET_DIR}/sources/ui/elementinfowidget.cpp