[BUGFIX] Fix wrong test in method registerProviderExtensionKey

According to the method definition $extensionKey is given as string.
Assuming, the logic is to search an extension key from an array of
extensions, the test must be fixed solving a warning on the FE.
Merge pull request #136 from fudriot/patch1

[BUGFIX] Fix wrong test in method registerProviderExtensionKey
[TASK] Remove unused code
[BUGFIX] Fix Warning if configuration wizard is empty

It may happen configuration wizard is empty. In this case, method
xml2array will return a debug string telling something went wrong. Since,
later in the code, it is assumed the output of xml2array is an array, a PHP
warning will be raised on the FE.
Merge pull request #137 from dreipunktnull/master

[TASK] Code cleaning
Merge pull request #135 from fudriot/master

[BUGFIX] Fix Warning if configuration wizard is empty
[BUGFIX] Add required uid field to select statement
Merge pull request #138 from dreipunktnull/master

[BUGFIX] Fix inheritance in DataViewHelper
[BUGFIX] Always add "clear value" wizard
Merge pull request #139 from dreipunktnull/master

[BUGFIX] Always add "clear value" wizard
[BUGFIX] Inserting element after another in content area sorts wrong

Fixes: https://github.com/FluidTYPO3/fluidcontent/issues/55
Merge pull request #140 from NamelessCoder/sortingafter

[BUGFIX] Inserting element after another in content area sorts wrong
[FEATURE] "Is reference" indicator and link to anchored original

Allows straight-to-position jump to original element if current shortcut-type element is a reference to a single content element - which is how Flux stores references.

Fixes: #41
Merge pull request #141 from NamelessCoder/reference

[FEATURE] "Is reference" indicator and link to anchored original