MNT: Common table views, Primarily LivePVTableModel, minor skeleton work on NestableTableModel

[tangkong]

Description

Build a couple table models that can hopefully be reused elsewhere. Includes:

• LivePVTableModel: a model for displaying {Parameter, Readback, Setpoint} data alongside the current values
• NestableTableModel: a model for {Collection, Snapshot} entries, which shows title / description primarily
• BaseTableModel that holds their own entries

Also:

• refactors a bit of the existing table models
• Adds a _PVPollThread for grabbing data outside the GUI thread

Note: I renamed superscore.widgets.tree -> superscore.widgets.views, but the diff viewer doesn't pick that up. I did not change the tree model at all, so the reviews can focus on the table models

To-do (in a later PR at this point):

• make columns hide-able (see GUI: LivePVTableModel additional features #76)
• Refine columns (also showing live status and severity? This might wait on ENH: Record status and severity when reading PV values from EPICS #71)
• Hierarchical headers to condense views (see GUI: LivePVTableModel additional features #76)
• Finesse poll callbacks and signals to minimally update the table

Motivation and Context

Building screens, I fell down this rabbit hole and the diff warranted a separate PR

How Has This Been Tested?

Interactively so far, a couple tests added

Where Has This Been Documented?

This PR

Pre-merge checklist

• Code works interactively
• Code follows the style guide
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[tangkong]

I may work on #71 first... these tables want those data types, and I'll need to rebase this branch anyway

[tangkong]

This got too long, so I'm cutting it off at the LivePVTableModel and leaving the NestableTableModel for another PR. There's also still some improvements to be made for this table model but maybe in the spirit of an MVP we put that off

[ZLLentz]

I think this was all nitpicks and questions, there isn't anything bad here
Feel free to discard the ones you disagree with

[ZLLentz]

What is the data class bridge used for with EntryItem? I remember its usage in atef but I don't see any other references to bridge here.

[tangkong]

We haven't used this in superscore yet, but its usage is intended to be similar to atef. At some point we'll probably update an entry and want to make sure that updates in the tree view.

[ZLLentz]

Is None preferable here to some sort of disconnected/not connected yet EpicsData value?

[tangkong]

This wasn't entirely intentional, None was just the easiest for me to initialize at the time. It might helpful to be able to differentiate between the not-yet-attempted state and an attempted-but-failed state, but I'm not wedded to it.

I suppose it might be better to keep the types consistent though

[ZLLentz]

LGTM