The PlasmaComponents2 TextField can draw its own Clear button, but the PlasmaComponents3 version can't, which impedes our ability to port code to use it without losing features.
This patch implements the same built-in Clear button in the PlasmaComponents3 TextField (code is basically a copy-paste) so that it will be a drop-in replacement for the PlasmaComponents2 version for text fields that need a clear button.
BUG: 396828
CCBUG: 396813
FIXED-IN: 5.49