Add DiagnosticsManager, refactor DiagnosticPanel

[bollwyvl]

Elevator Pitch

Create a DiagnosticManager singleton. Move DiagnosticsPanel into its own file (and top-level plugin #163).

Motivation

The diagnostics are one of the message types that "just happen" from the backend, and so cannot be Promise-ified (#165). Having a single manager means there would only be a single listener per connection event (or whatever we end up using), and then we can use a Model<Document> for each documentUri which can subscribe to them.

With a central store of the last-seen diagnostics, the panel could show all diagnostics, even if initially filtered to a specific file (#160), and you could see warnings between multiple files.

Design Ideas

   + Show the icon/count for the highest level of thing found
   |                          
|-------------------------| 
| [E] Diagnostics 2       |________________________________________________
| File [Current V]  Search [           ][.*] [copy] [filt 3] [cols 3] [x] | -+ hideable tool bar, toggle between current/all    
|-------------------------------------------------------------------------|   - search is dumb/regex (across everything)
| [!] | Message              | Location           | Code  | Source | Lang |     - `<field>:thing` modifiers
|-----|----------------------|--------------------|-------|--------|------|   - copy all/selected
| [E] | Bad code             | Untitled.ipynb:1:1 | E302  | pyfoo  | [py] |   - persistent filters (with count hidden)
| [W] | Bad Whitespace       | Untitled.ipynb:1:1 | W100  | rbar   | [r]  |   - cols (with count hidden)
--------------------------------------------------------------------------|
  |                                     |                             | 
  |                                     |                             + icon (kernel, file, or override in server spec)
  |                                     + collapse previous columns, support "Go To" by file/line/col
  + icons (a la gutter)

[bollwyvl]

Also ref

• Diagnostic Panel Action Column #160 this would be enough value to warrant having a (hideable) toolbar
• Allow to copy the diagnostics message from the Diagnostics Panel #154 from the toolbar, be able to copy all/selected diagnostics

[krassowski]

I very much like the ASCII art design. Also agree on splitting the diagnostics panel into a separate file.

Implementation-wise I would probably prefer to keep the Feature class hierarchy for simplicity, but this could be supercharged to allow singleton-like behaviour (in the sense that only one connection would be opened). We could have a parameter/subclass for such Features.

These are just initial thoughts...