Klipper (classic widget): Use standard colour scheme to show filter errors

Instead of having Qt::red hardwired for the text colour.

New function KlipperPopup::showStatus() to implement setting
the filter line edit colours and showing the status.
Show an explicit error for an invalid regular expression.
Simplify filter regexp generation, add comment regarding case sensitivity.
No need to have single use strings as member variables.

I18N: