Metadata download: Do not auto trim downloaded covers

Trimming can sometimes have undesirable effects.
Conversion: Treat .docm the same as .docx files, ignoring any macros in the file. Fixes #1247565 [Calibre does not recognise Word files in docm format](https://bugs.launchpad.net/calibre/+bug/1247565)
Limit size of savepoint history
Allow launching Tweak Book easily via calibre-debug
Implement saving for individual editors
Closing of editor tabs
Go back to help screen when last tab is closed
Nicer error message when failing to open exclusive file
Close editors when files deleted
Refactor simple worker creation
A simple worker to run function calls in a separate process
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Booklist: Fix using Page Up/Down keys moving book list by one row too many. Fixes #1248109 [Page up and page down keys scroll one book too much](https://bugs.launchpad.net/calibre/+bug/1248109)
Remove log file when shutting down offload worker
Handle case of less than a screenful of books
Update New York Review of Books
Book list: Preserve the current column when using Ctrl+Home or Ctrl+End shortcuts
Start work on preview panel
Implement loading of resources direct from editrs->preview pane
Ignore links clicked in the preview panel