[FEATURE] Leaving edit by clicking in page tree does not show a warning

If edit forms have unsaved changes, changing the IFRAME URL is caught by
explicitly sending interaction requests that are handled by individual
client components, such as the FormEngine. This feature does not use the
Window.beforeunload event, but some custom messaging API instead.

Click events on the ExtJS page-tree are caught if those trigger a change
request for the content component, however highlighting the clicked page
node is not caught due to nested ExtJS event hierarchies.

Resolves: #77268
Releases: master
Change-Id: I3e2359cf27d95197b17e8d8489759ace403ce1af
Reviewed-on: https://review.typo3.org/53075


Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Jasmina Ließmann <code@frauliessmann.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>