Notifd KDE inline reply extension support

[Mabi19]

Is your feature request related to a problem? Please describe.
It'd be neat if AstalNotifd supported the non-standard KDE inline reply notification extension.
When the extension is unsupported, apps put "Reply" buttons that spawn a temporary window, which I find awkward.

Describe the solution you'd like
Support for inline reply in the AstalNotifd API.

Describe alternatives you've considered
Doing it manually by reading hints - unfortunately it requires advertising the inline-reply capability and emitting a different signal, so it can't be done without modifying AstalNotifd

Additional context
This isn't documented, I got this information from reading the KDE patch that introduced the feature: https://phabricator.kde.org/D25851

Supporting inline reply involves the server advertising the inline-reply capability and emitting the NotificationReplied D-Bus signal (instead of ActionInvoked) with id (as in the notification ID) and text parameters, and also parsing a few string hints: x-kde-reply-placeholder-text, x-kde-reply-submit-button-text, x-kde-reply-submit-button-icon-name. Then an action with the ID inline-reply should show an inline entry field instead.