Quick opened files should be kept open, not previewed

[bgashler1]

• VSCode Version: 1.4.0
• OS Version: macOS

Desired behavior

When you use Quick Open (Cmd + P) to open a file, that file should stay open (i.e. it shouldn't be a preview tab).

Current behavior

Current behavior opens it as a preview tab, which can be undesirable. If you open a lot of files up front through quick open, you have to double click the tab or run command "Keep Open" for each file. It makes it very slow to open a lot of files from Quick Open.

Since some people may want to rapidly preview a lot of files, then we should do what Sublime does in their version of Quick Open: upon keyboard focus or hover of any file in Quick Open, a preview tab is shown. Only when you press Enter or click does it keep it as an editor and close the Quick Open.

If we click away from or press Escin Quick Open, then this preview tab would close, unless there already was a preview tab open before using Quick Open (in that case the previously previewed file would appear as if nothing had ever happened, so we don't leave a mess).

[bpasero]

The setting workbench.editor.enablePreviewFromQuickOpen controls this. Otherwise this looks like a duplicate of #8939