ENH/REF: Modify FillTemplatePage to have staging area

[tangkong]

Description

• Adds a staging area to FillTemplatePage, to be more clear about which changes are being queued for application
• Refactors various helpers and widgets to support this
• Adds tree-view that highlights closest tree-item when a row detail is examined

Motivation and Context

In looking into reusing FillTemplatePage for templated checkouts, it became clear that the page needed a refactor

• If we were to serialize actions, we would need some way to display those queued actions.
  • Previously, FillTemplatePage would maintain an internal "all actions" list and a user could apply them gradually. This made the current state of the loaded checkout ambiguous (had we applied changes yet?)
• Edit actions (FindReplaceAction) saved in a templated checkout would need to be loaded without knowledge of the search query that produced it.
  • Prior to this FillTemplatePage would gather all actions in any populated edit when asked to apply all. The user would either have to know that these were the right actions or click through them all to verify
  • Now the staging area makes clear what will be applied when we save the file, and edits are not applied until the new file is saved

How Has This Been Tested?

Entirely interactively so far. But the test suite passes

Where Has This Been Documented?

This PR.

Compare to before:

Details

Pre-merge checklist

• Code works interactively
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Code has been checked for threading issues (no blocking tasks in GUI thread)
• 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

[ZLLentz]

I always find it really hard to review atef
I had a comment and a docs nitpick
I'll try to run this myself to play qa bugfinder

[ZLLentz]

Was this unknowingly broken for a few days?

[tangkong]

Yes 😢 . My ctrl+f did not ctrl+f enough it seems

[tangkong]

I always find it really hard to review atef

I'm not sure how to make this better. atef is complicated but I think the web of signals and slots tends to make things opaque.

I've often wondered what it would take to make the atef development process more clear...

[ZLLentz]

Design thoughts and nitpicks:

• The "select file" button is surprisingly hard to find from a blank template fill because it's at the bottom of a very long screen and selecting a file ends up populating the very top portion of the screen. I wonder if there should be another way to open a file, some clicky click magic in one of the top areas, for example.
• What is the intent of the "signals" column in the top-right view? It certainly isn't showing device signals, is it just for PV configs? If it's for PV configs I suggest marking the column as e.g. PVs instead of as Signals to evoke the same terminology as used in the rest of atef.
• I'm glad you have the movable dividers because qt default resizing is pain as always. One of these is slightly awkward though because it's so so close to the "Stage All" button, it's easy to click the button by mistake while aiming for the divider.
• I didn't see any bugs
• The "edits" table header is the only text element on the display that isn't capitalized (should probably be "Edits")

[tangkong]

"no obvious bugs" has to be an all-time high for me 😆

[ZLLentz]

I'm 👍 on this design update

[tangkong]

Great, thanks! I'm going to merge this and forge ahead with the GUI elements that fold this into the main checkouts.