Bug 19267: Rancor - Warn before leaving page if record modified

Bug 19267: Rancor - Warn before leaving page if record modified

To test:
1 - Open the advanced editor
2 - Make some changes to a record
3 - Click on 'Patrons' or leave the page in some way
4 - Note there is no warning and changes are not saved
5 - Apply patch
6 - Open the advanced editor
7 - Make some changes
8 - Leave the page
9 - Note you get a warning of unsaved changes and can stay on the page

Signed-off-by: Charles Farmer <charles.farmer@inLibro.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 5a6e7a6ffbab8e42e706486b9e75abe782a696b8)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>