[containments/panel] Don't close pop-up unnecessarily

We unset the currentApplet variable when the cursor leaves the area of an applet, and the contextual applet pop-up will close if it's open.

Unfortunately, since panels have margins, if the cursor happens to pass over those margins on its way to click on something in the pop-up, currentApplet will get unset and the pop-up will close as soon as the mouse reaches the thing you wanted to click on! This problem becomes worse for applets using expanded margins due to the use of a Margins Separator applet, or Plasma themes with their own expanded margins.

To prevent this frustrating situation from happening, we no longer unset currentApplet when the cursor leaves the area of an applet; instead we only reset it to something else when the cursor passes over a new applet.

BUG: 413736 FIXED-IN: 5.18.7