Show page size field in Properties dialog when there are multiple sizes

Currently when a document has multiple page sizes, the Properties dialog
does not show the "Page Size" field at all. This MR populates the field
with the string "Multiple" for documents with multiple page sizes.
1 job for master in 17 minutes and 5 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #48189
linux kf5-qt5 qt5.12

00:17:05

43.0%