NPP should be open even when no markers are found

[marcel0ll]

Current Behavior:
NPP hides itself when there are no markers found

Expected Behavior:
NPP keeps open even when there are no markers and shows a message: "No markers found"

[johnarwe]

I had considered this too; no objection, although I don't see it as high enough priority that I'd likely work on it any time soon. It would be "convenient" (a good design point?) when making this change if the result consumed minimal screen space. To throw out a couple of possibilities:

1. Use just the panel header area (where the filter box normally lives); don't want to overlay a fair chunk of the right side with a blank grey box, doing that would waste data-column width.
2. Have the message be similar to the output of editor:log-cursor-scope, except that the message box self-deletes after a small number of seconds. Both toggle() and populatePanel could safely use this, I suspect.

Both amount of screen real estate consumed and the length of said consumption matter. I tend to ask myself "what if every package did this?" before going too far, especially given Atom's stated preference for "lots of small packages" each adding a discrete feature.