Add a SearchField and PasswordField component

Summary:
Provide a small abstraction over ActionTextField to implement consistent standard special text field

Warning: I include a placeholder text in this two example, not sure if this is correct, because kirigami probably shouldn't include translatable content.

Also improve the ActionTextField doc

Test Plan: Compile, example works

Reviewers: Kirigami, ngraham, mart

Reviewed By: Kirigami, mart

Subscribers: broulik, apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D19502