Stream browser dialog DMM support

[fredzo]

Hi Andrew,

Here is the second part of DMM support for StreamBrowserDialog (see first part here: https://github.com/ngscopeclient/scopehal/pull/942). I created a dedicated 7 segment-style display to mimic real multimeter display (implemented in Dialog class to be reusable in other windows). There is a setting to chose between classic display and 7 segment display for DMM and PSU values:

Best,

Frederic.