Improve message history browsability

[alex-bermishev]

In "raw value" mode Value preview window does not show content of History messages, when clicked on them
In "show difference..." mode Value preview works property - shows difference between message clicked in History list and the last received message

[thomasnordquist]

The value preview is intended to show the last received message,
the history was intended to see differences to the last received message.

I take your expectation on how the UI should work under advisement, historic messages are currently "hard" to browse.

[thomasnordquist]

Compared to other solutions, mqtt explorer does a bad job reviewing previous messages.
Should be a priority.

[chaholl]

Can I suggest a minor tweak to 'raw' mode that will make historical messages easier to read?
At the moment, in raw mode, the current value is shown, with the selected historical value below. This is just fine for simple messages. However, if you have a JSON payload, you end up scrolling all over the place to read the data because there are two versions on the screen.

Since we already have the ability to view a diff, we probably don't need to see both values in raw mode. If we show only the current value if no historical message is selected, or the selected message, we'll use less screen space and make complex values easier to read.

I'll be more than happy to submit a PR with this change if it's likely to be accepted.