Introduce FileObject#isUploadable for files about to be modified

When users want to edit metadata in the updater GUI, we ask the file
object whether it can be uploaded. However, in this case the file's
metadata is about to be changed, but not yet changed. So we need a way
to ask: *could* this file be uploaded if the metadata were changed?

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Avoid using the now-deprecated isUploadable(FilesCollection) method

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Changed metadata constitutes a change, even an uploadable one

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>