Bring corner radius of popup messages in line with widget style

In 0070baaf `PageViewMessage` to display on-canvas notifications was
introduced, featuring rounded corners. Over the years, overall
preferences in the industry regarding the corner radius have changed
multiple times, with recent efforts trying to ensure a more consistent
usage in the entire UI of KDE's products.

Let's use a corner radius better matching the radius commonly used in
the current widget style, i.e. a bit less rounded, to make the message
fit in better with surrounding buttons and frames. Note that this is not
specific to Breeze, since most widget styles nowadays use a similar
corner radius. The radius is compatible with `KTextEditor::Message` too,
which is used in a similar context.

Test Plan:
  - Start Okular: The "Welcome" message and the "Document Loaded"
    message have a more professional look to them.
  - Open the annotations sidebar: The "No Annotations" message better
    fits in with the sidebar's frame.