Replace KStandardAction::PasteText with KStandardAction::Paste

The PasteText standard action creates a KPasteTextAction rather than
a simple QAction. KPasteTextAction is a very old class that has been not
functional for years. Since this action is already used as if it actually
were a QAction, just use KStandardAction::Paste which is what everyone
else does.

See also https://phabricator.kde.org/D7294

Differential Revision: https://phabricator.kde.org/D7391