SharePlugin: Read and write shared file creation time

When sharing a file, add metadata about the file's creation time and last modified time. When receiving a shared file, read those fields and set them on the file we create.

Before this change, we would write the last modified timestamp when receiving files, but not read it when sending them.

Note: Qt doesn't support setting the creation date on Unix [1].

[1] https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/io/qfilesystemengine_unix.cpp?h=v5.15.2&id=40143c189b7c1bf3c2058b77d00ea5c4e3be8b28#n1590