Add a widget bar with a label in the center

[Zannick]

This isn't quite "nested widgets" but it accomplishes something I wanted to do with the progressbar: put the percentage in the center of the bar instead of finding space for it to the side.

• Adds a widget FormatLabelBar that accepts a format string and constantly centers it.
• Adds a widget PercentageLabelBar that puts the percentage in the center of the bar. Includes a test case and an example.
• Moves the N/A% check in Percentage somewhere it can be used by a descendant.
• Allows 0% to be shown instead of N/A% when the percentage rounds to 0. (This is apparently all the test cases for it.)

Possible improvements: allow justifying the label on the left or the right, i.e. determine the left or right position of the label within the center based on its maximum width, so the label doesn't "wobble" as it changes.

[lgtm-com[bot]]

This pull request introduces 2 alerts when merging 67a8889167cbb520f70b68ba10782b3743576f0c into ff05b64131234ffc05707c266e386dfdca63d109 - view on LGTM.com

new alerts:

• 2 for Multiple calls to `__init__` during object initialization

[Zannick]

I suppose based on that I should reorganize the class structure a little so the label isn't dependent on width.