Extract header Panel to Display::Panels::Header .

Instead of defining each `Panel` within `Display::Screen`, extract to
`Display::Panels::Header`, inheriting from `Display::Panel`. Also make
height of header explicit; the defined structure shouldn't overrun this,
but if it does, it won't be displayed.
Remove dividers from Translatable .

It is, perhaps, simpler to define dividers in `Display::Panels::X`
instead of passing through from `Translatable`. Which (if any) dividers
to use is part of the design of the panel. One option would be to define
dividers per-`Panel` in `Translatable`; however, if this functionality
is required, the dividers can again be extracted.
Split header panel into header,redis_stats,sidekiq_stats .

Slim `Display::Panels::Header`, creating `Display::Panels::RedisStats`
and `Display::Panels::SidekiqStats`. Attach in `Display::Screen`. This
removes the need for specifying `nil` rows as spacers. Also remove
spacers from header panel.
Merge branch 'display-refactor'