feat(ui): improve "Example Queries" popup to support groups

[argshook]

This PR updates the "Example Queries" popup with a support to
display queries in groups.

For example, queries related to geohashing can be under a single group.

Depending on savedQueries array in
assets/console-configuration.json, the popup can look like so:

the savedQueries array in assets/console-configuration.json is what
governs this list. The array for what's seen in the screenshot is
roughly this (truncated for brevity):

{
  "savedQueries": [
    {
      "title": "Trading",
      "description": "Examples of crypto currency trading queries",
      "queries": [
        {
          "name": "All BTC trades",
          "value": "select * from trade where symbol = 'BTC'"
        },
        // ...
      ]
    },
    {
      "title": "Geohash",
      "description": "Queries for geohashes",
      "queries": [
        {
          "name": "Current positions within a geohash area",
          "value": "pos latest by id where geo6 within(#wtq);"
        },
        // ...
      ]
    }
  ]
}

This PR ensures that json schema used previously still works well. This
kind of assets/console-configuration.json still works just like
before:

"savedQueries": [
  { "name": "some name", "value": "some query" }
]

Both schemes can work together, this PR really just makes previous
schema nestable, so we can do this:

{
  "savedQueries": [
    { "name": "some name", "value": "some query" },
    {
      "title": "some group",
      "description": "this is a group of queries",
      "queries": [{ "name": "query in a group", "value": "some query" }]
    }
  ]
}

Just a tree, traversed in BFS style