Reset the hide timer if a new inline Message is displayed

Summary:
Previously the KMessageWidget was unconditionally hidden after 10 seconds after it became visible. Even if during
this timeframe a new message was shown. This correctly stops/resets the timer if another message should be
displayed while the widget is shown.

Test Plan:

• Click on "Copy to Clipboard"
• Wait 5 seconds
• Click on "Save"
• The widget hides after 10 seconds instead of previously 5

Reviewers: Spectacle, ngraham

Reviewed By: Spectacle, ngraham

Subscribers: ngraham, broulik

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D21352