Improve usability of demo extension

[deboer-tim]

It feels odd to open an issue for a feature that hasn't merged yet :), but since I am not expecting these changes in PR #27 I wanted to track my thoughts somewhere:

• The tooltip message telling the user to run preload wasn't obvious enough (I didn't even realize that we had tooltips on the statusbar), it should be part of the error messages and say init.sh.
• If it hasn't been run yet, one error dialog is enough, don't need two. ( podman-desktop-demo.ts:312 should exit if the pull command failed so that you don't get two error dialogs)
• The icon shouldn't change until it does something.
• Even more bonus points: we should open the repo's readme, or a page on what the extension does and suggested demos (e.g. resources for external demos/tech sales teams).
• The action failed until I restarted Podman Desktop, should investigate why.