Improve Dismissing Welcome Page

[tsmaeder]

Feature Description:

While dog-fooding, I thought I had found a bug: #277. It turns out, this works as designed. However, I would suggest we improve the experience in this area:

1. Make the "off" the default. I would argue for most people it's just annoying (even thought VS Code has the same behaviour). Whenever the welcome page changes (like after an update), we should show it again (once).
2. Move the UI for turning off the welcome page to the top (or make it otherwise more obvious). VS Code shows the checkbox in the middle of the page instead of left-aligned, which makes it stand out a bit.
3. Call it "Welcome" not "Get Started". VS Code calls it "Welcome" and extensions like Gitlens call their welcome pages "Welcome" as well. There is no value to being different here. When I try to get the welcome page back via the command bar, I didn't find it at first.

I am aware that these are nitpicks, but the welcome page is the first thing a user sees. We should make a good first impression.