List `xbeam` improvements

[vk2seb]

Grab-bag of things that would make xbeam more useful and less of a toy:

• Allow triggering from any channel except for only Ch0
• Add optional gradations / grid
• Switch timebase to enum that doubles on every tick like a normal scope
• Re-add optional intensity and color modulation to xbeam vectorscope: These used to be present but it was not clear how to handle the default intensity. Perhaps a fixed offset that defaults to full brightness in the menu would be a good answer.

[vk2seb]

• Slowest time base is too fast for looking at envelopes
• Y deflection on scope is inverted - make polarity an option (and default correct)
• 8.192V clamping makes the scope less useful for 10V envelopes
• Trigger can be unstable while menu is being drawn - don't block the trigger stream when - memory accesses block drawing

[vk2seb]

Most of this was fixed in #120

Still open is:

• 8.192V clamping makes the scope less useful for 10V envelopes
• Add optional gradations / grid