Fix inter-line deadzone in hexdump view

[therealryan]

You can roll the pointer over the bytes displayed in the hex view (for example) and the hovered value is explained at the top of the dump.

The transition between side-by-side bytes is fine - there is no gap so the explanation flicks from one value directly to the next.
The transition between one-above-the-other bytes is less good - there's a wee inter-line deadzone where the explanation flicks to blank. This is very minor but kind of annoying - it'd be nicer if the bytes were displayed as a seamless grid with no inter-line gaps in the hover coverage.

[therealryan]

While we're at it we should look at the inter-line gaps in the message views - they make box-drawing characters less useful. e.g.:

┌───┬─┐
│   │ │
├───┼─┤
└───┴─┘

should be displayed without gaps between the vertical elements

[therealryan]

For the message view: line-height:normal does the trick