[FEATURE] htmlArea RTE: Add button to insert soft hyphen

This feature adds a button and/or shortcut to insert a soft hyphen at
the current cursor position in the RTE content.

This feature was sponsored by BLE (Bundesanstalt für Landwirtschaft
und Ernährung).

Change-Id: Id87fcde001b042ff6d0423bf3135d741aa3f2a4f
Resolves: #33184
Releases: 4.7
Reviewed-on: http://review.typo3.org/7787
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland