the gtk-cancel button overlapped the "adding extension" label if %EXTENSION_NAME is too long.

Change-Id: Ia6a650d7ff82853155da67527a1cdc865b01148e
Reviewed-on: https://gerrit.libreoffice.org/14717Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>
Tested-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>