Change the save bar to use KMessageWidget

Benefits:

• Uses the standard KMessageWidget, not a bunch of custom code and bespoke CSS. Negative diffs!
• Unrelated to visual changes, the "Save All" is only shown when there's actually more than one file.

Here's a visual representation of what it might look like:

Save Bar Only

Before	After

Save Bar + Full Memory Bar

Before	After

Save Bar + Full Memory Bar (Fullscreen)

Before	After