Stream browser dialog upgrade

[fredzo]

Hi Andrew,

This PR goes along with ngscopeclient/scopehal#1064 of scopehal.
Here are the changes:

• Adds drag and drop capabilities to Digital Bank nodes in StreamBrowserDialog (fixes Feature request: add a whole Digital Bank at once to a Waveform Group scopehal#1052):
  • Drag digital bank to center left drop area of WaveformArea (all channels from the bank are added to area)
  • Drag digital bank to center right drop area of WaveformArea (creates one new area for each channel)
  • Drag digital bank to edge drop area: creates a new Waveform window with a WaveformArea for each channel (or one WaveformArea with all channels if shift is pressed when dropping)
• Adds "Enable/Disable" toggle to oscilloscope channel nodes,
• Adds "Enable/Disable" toggle on Digital Bank nodes,
• Fixes "On/Off" toggle on PSU node.

[fredzo]

As discussed with @azonenberg , the Enable/disable toggle on Oscilloscope channels and Digital Banks has been removed.
Currently, channel enable state is tightly coupled with reference counting.
For that reason we cannot let the user enable/disable channel manually.
This feature can be restored once we have completed the reference graph rework.