Since 3.32.0, I've noticed that modal-dialog's shadows are truncated like
PopupMenus. But in this case, we can't move shadow parameters to 1st child
elements like PopupMenus (Because ModalDialogButtons are directly embedded
into root .modal-dialog class for example).

This commit introduces damned extremely ungly SVGs to draw drop-shadows.
I really wanna drop them ASAP if upstream solved the unwanted box-shadow
truncations.

This also might introduce some side-effects like missing-backgrounds,
incorrect background sizing, and incorrect dialog corner radius on
some types of modal-dialogs (I didn't verify all types of modal-dialogs yet).

Anway this should fix the shadow truncations on run-dialog and end-session-
dialog at least...