Edit Book: Allow editing xpgt (Adobe Page Template) files as XML.

Fixes #1346543 [Edit Book: Allow editing xpgt files](https://bugs.launchpad.net/calibre/+bug/1346543)
Edit Book: Make the undo/redo/cut/copy/paste buttons on the editor toolbar also configurable, so they can be removed via Preferences->Toolbars, if needed.

See #1346913 (Edit Books - Toolbars)
Edit Book: When re-arranging the toolbars inside an individual file editor, save the new toolbar layout so that it is re-used in the future.  Also apply the changed layout to all open editors of the same type. See #1346913 (Edit Books - Toolbars)
Edit Book: Fix close event geenrated by window system not saving state
Edit Book: Make the warning dialog that pops up when you try to quit while the editor is saving changes in the background a little more clear. Fixes #1347171 [Background save dialog confusion with "save changes"](https://bugs.launchpad.net/calibre/+bug/1347171)
calibredb: When listing formats of a book if there are multiple formats, separate the individual formats with the colon instead of the comma as commas can occur in filenames
Book details panel: Automatically convert ISSN identifiers into links to the periodicals page on worldcat.org.
calibredb list: Do not output extra whitespace+separator after the last field
calibredb list: Add a --for-machine option that causes the output to be made in a form more easily parsed programmatically (it uses ASCII field and record separators)
Fix published date not being merged when merging book records
typo that prevented an optimization to PDF metadata update when nothing changed
Update manual text to match new button layout
Fix #1348054 [Private bug](https://bugs.launchpad.net/calibre/+bug/1348054)
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Add documentation and an example editor plugin
Conversion: Fix a bug processing the CSS font shorthand property when the line-height is specified in em or percentage units. Fixes #1348173 [ebook-convert doesn't handle composite font-sizes](https://bugs.launchpad.net/calibre/+bug/1348173)
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calibre-customize: When building plugins from a directory, exclude known version control subdirectories
version 1.46.0
Fix regression that broke the book details panel when displaying books whose authors have a manually specified link