Cancelable Save (to File) Command

[pfleck]

After the improvements in #2845, we should make the "Save Item"-progress cancelable. This is especially helpful for large files when someone accidentally pressed Ctrl+S.

However, we first need a way to cancel the serialization progress, therefore the persistence has to be slightly adapted, by passing a CancellationToken into the corresponding methods.

This ticket has to be released after #2845.

[pfleck]

r16321 branched trunk

[pfleck]

r16325

• Added CancelationTokens for the Save and Serialize methods.
• Fixed a potential temp-file-leak when replacing the old file with the new one after serialization.

[pfleck]

r16433 Merged recent trunk changes. Enabled the prepared hooks that allows to cancel the save file using the recently introduced cancelable progressbars (in FileManager).

[pfleck]

r16439: Merged trunk changes into branch.

[pfleck]

r16440 merged branch to trunk

[mkommend]

Reviewed r16321, r16325, r16433, r16439, r16440.

[pfleck]

r16931 removed branch

[mkommend]

r17068: Merged r16440 into stable.

[gkronber]

r17162: add info that r16496 has been merged from trunk to stable (in r16580)