postmarketos-demos improvements

I got some ideas after meeting issues with postmarketos-demos usage on Finepower F1 which I'd like to discuss here.

1. Change the UI so the window fits the screen. I had no ability to reach bottom part of the window, so I couldn't start some tests. Possible solutions:

• Make list of tests scrollable;
• Make different layout: horizontal grid.

2. Add/remove some tests. For example, Firefox item is useless on most devices, so it could be replaced with any other browser.

3. Rename postmarketos-demos to postmarketos-debug-tests (or something like that) and move it out of default installation. If more tests are to be added, there should be additional packages to be installed like hwtest, motion, gpsd and others.

I don't know if I can assign myself to this issue, but I want to work on it. Any feedback appreciated.