What's the name of this icon ?

It's a Tool window quick access button. From https://www.jetbrains.com/help/pycharm/2016.3/pycharm-tool-windows.html#tool_window_quick_access:

In the lower left corner of the workspace, there is a button which initially looks like this . If you hover the mouse cursor over this button, a menu opens that provides quick access to tool windows. If you click this button, tool window bars and buttons are shown. At the same time the button, appearance toggles to . If you click the button again, the tool window bars and buttons are hidden again.

thanks