Add a link to the editor video tour in the User Manual
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Simplify code to show GUI debug log

Fixes regression caused by Qt 5 behavior change that prevented the
debug log from being displayed in linux.
Workaround for bug in Qt 5 that causes crashes if QDesktopServices is used before constructing a QApplication
Abort terminal save on error instead of quitting editor
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PDF Output: Fix balanced parentheses in the text of items in the Table of Contents not being rendered properly by some PDF viewers. Fixes #1358443 [PDF ToC omitting entries in parentheses](https://bugs.launchpad.net/calibre/+bug/1358443)
E-book viewer: Remove extra spacing between document and scrollbars.  This is particularly noticeable when using a dark theme for the viewer.