core:
 * Fix rendering of some extended latin1 characters in annotations. Issue #1070
 * Support some not so well formed documents with password. Issue #1083
 * Add API to get notified if the xref is reconstructed
 * Add somewhat fancier left/right signature visual representation
 * Fix crashes in malformed files
 * Minor code improvements

qt6:
 * Change some functions to return std::unique_ptr

qt5/qt6:
 * Add API to get notified if the xref is reconstructed
 * Add somewhat fancier left/right signature visual representation
 * Don't assert when trying to invert singular matrices

build system:
 * make boost opt-out if building splash