Include a bottom toolbar for the application page

Summary:
After a chat with Jens, we decided to introduce this toolbar.
We've wanted to have a back button for a while and we never liked the floaty
button with a shadow. I find this solution much more elegant.

Reviewers: leinir, colomar, jensreuterberg

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5339