Add more whitespace to the status bar

Summary:
This patch gives the status bar a little more vertical whitespace, prevents the status bar from "jumping up and down" when showing a progress bar, and adds a bit of padding on the sides so the labels are more clearly separated when using dual panes.

Reviewers: Dolphin, VDG, ngraham, elvisangelaccio

Differential Revision: https://phabricator.kde.org/D8540