[applets/touchpad] Fix potential undefined property access

There's a race condition between the hasTouchpad and touchpadEnabled properties.

This commit fixes a potential undefined property access, in case if an expression for touchpadEnabled property is re-evaluated before its dependency -- hasTouchpad property -- toggles from true to false.

With this commit patch applied, there's still a race in Plasmoid.status and Plasmoid.toolTipSubText, but we don't care about it as much, as those harmless strings/enums would become "eventually consistent" by the end their cycle of an event loop.

CC @broulik