Until Plasma 5.4.2 (wily) widgets could be unlocked and showed a side menu when hovering over them with the mouse. The side menu allowed moving, resizing and removing the widget. With Plasma 5.5.5 (xenial) the side menu does not appear anymore.
SnapOverflow
With Plasma 5.5.5 the menu appears after a long left-click (instead of hovering with the mouse in Plasma 5.4.2). This can be changed back in Desktop Settings (right click on desktop) -> Tweaks -> Widget Handling. This solution was first posted on bugs.kde.org.