A small grey square resides in the middle of my Lollipop screen
Since some days a small grey square resides in the middle of my Lollipop screen (left side). I may have used an app or sg, a kind of an accessibility tool? when I tap on that square, a shortcut screen pops up, with the four default apps.
How can I configure this feature? may I select other apps as well? and how to switch the whole off?

Since some days a small grey square resides in the middle of my Lollipop screen (left side). I may have used an app or sg, a kind of an accessibility tool? when I tap on that square, a shortcut screen pops up, with the four default apps.
How can I configure this feature? may I select other apps as well? and how to switch the whole off?

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