Add command for showing all class members, including inherited

[isc-bsaviano]

This PR fixes #1623. Adds a class icon button to the editor title bar when the editor contains a class. When clicked, a QuickPick appears with all members of that class, including inherited ones. Clicking on a member shows it in the editor.

I initially tried to implement this as a tree view, which prompted me to make some architectural improvements. I kept those in because I think they are worth including. I consolidated three three onDidChangeActiveTextEditor handlers to a single one, and I added the existing tree views to the array of disposables so their resources get cleaned up properly.

[gjsjohnmurray]

Very nice. The way the symbol-class icon displays colored in the editor's action bar looks a bit weird, but that's not in our control. I'm going to open a VS Code issue about it. Approving this PR.

[gjsjohnmurray]

Perhaps the changelog for this item should mention that the Show Class Documentation Preview command has been dropped from Command Palette and from the editor tab's context menu, but can still be found on the editor's action bar.

[isc-bsaviano]

I will check to see if I can apply a color to that ThemeIcon. According to the docs the color is only used for TreeItems.

[isc-bsaviano]

When I write up the CHANGELOG I'll add a second line for that

[isc-bsaviano]

The way the symbol-class icon displays colored in the editor's action bar looks a bit weird, but that's not in our control.

There is no way for me to change the color of the icon in the command definition, so I will merge this

[gjsjohnmurray]

I will check to see if I can apply a color to that ThemeIcon. According to the docs the color is only used for TreeItems.

I think the CSS change in microsoft/vscode#170562 should probably have excluded the case where the codicons are used in monaco-action-bar elements, or maybe even in monaco-toolbar ones.