Request: reduce/debounce URL notifications

[wKovacs64]

Expected Behavior

• An empty URL should not trigger the invalid URL notification.
• A valid URL should only trigger the success notification once (until the input has changed).
• An invalid URL should only trigger the error notification once (until the input has changed).

Current Behavior

The notifications triggered by URL validation are a bit spammy. Specifically, when the URL input is focused, changing window focus will trigger a notification every time. After Cmd/Alt-Tab'ing a few times (to compare Altair contents with another window, for example), they stack up quite quickly. Additionally, an empty URL is considered invalid (understandably) and triggers the error notification (unnecessarily, IMHO).

Possible Solution

• Only fire the error notification if the URL input has data and is invalid.
• Only fire notifications if the URL input has changed.

Steps to Reproduce (for bugs)

1. Focus the URL input.
2. Swap Altair with another window a few times.
3. Observe stacked notifications.

Context

Obviously this isn't a big deal, just a small quality-of-life thing I noticed when I first checked out the app. I was going to submit a PR but I don't have experience with Angular or TypeScript so I'm not sure what all you've got going on here. :)

Your Environment

• Version used: 1.5.2
• Environment name and application type (e.g. Chrome 39, chrome extension v1.4.7): darwin-x64
• Operating System and version (e.g. Linux, v1.4.7): macOS 10.12.6 Sierra