Drop picture files on a folio to add them

Picture files (png, jpg, bmp, svg) dragged from the file manager onto a
folio are added there: the first centred on the drop point, the others
cascaded from it, and one undo step removes them all. A picture larger
than the visible part of the folio is scaled down to fit it. Files that
cannot be used are listed once after the others have been placed, and a
drop holding only other files (a .qet project) still reaches the main
window, which opens it.

The checks a picture must pass before it is embedded in the project
move into ImageDrop::load and are now shared by the drop, the add image
dialog and the script API: a regular file of at most 10 MB, and at most
64 megapixels, read from the header before any pixel is allocated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmake/qet_compilation_vars.cmake

@@ -216,6 +216,8 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/diagramposition.h
   ${QET_DIR}/sources/diagramview.cpp
   ${QET_DIR}/sources/diagramview.h
+  ${QET_DIR}/sources/imagedrop.cpp
+  ${QET_DIR}/sources/imagedrop.h
   ${QET_DIR}/sources/elementdialog.cpp
   ${QET_DIR}/sources/elementdialog.h
   ${QET_DIR}/sources/elementprovider.cpp