Use unicode ▄ and █ for the graphical statistics instead of "|" and ":"

[rbanffy]

There are a number of symbols that can be used for the graph with the reasonable expectation they'd be supported on most terminals currently available. This specific substitution should be relatively trivial. Further enhancements could be using the range between ▁ and █ for more precise graph or the upcoming Unicode 13 smoothed sextant chars for a slightly less precise graph that'd look a bit better.

The VTE folk is already working on supporting the UC13 symbols directly without reliance on special font support, but the downside is that these are not supported by older terminal programs.