[BUGFIX] Allow overwrite when uploading an existing extension

This patch adds a checkbox to the upload form, which makes it possible
to overwrite the existing directory when uploading an extension
which is already available in the system.

Resolves: #41006
Releases: 6.0

Change-Id: Ib51a10a0e557060f0260e97ff8a9129f1c9f7eba
Reviewed-on: http://review.typo3.org/15257
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn