Improve run instances UX: avoid removing data and add a clear popup.

[ckaiser]

Improves the experience of using the "Run Instances" dialog by not aggressively resizing the internal structures, I was bitten by this multiple times when testing things that require multiple instances with different parameters: If I have the dialog set up with 4 instances with different parameters but I want to run two instances temporarily for a different test, lowering the number deletes all the data in the rows that are gone, so when I go back to the old number I'd have to set it up again.

To make it a bit easier to remove things if, for example, the user creates a ton of different params and wants to go back, I've added a right click menu with the option to clear a single line in the tree or to clear everything, I think that should cover the more common use cases.

Here's a video of it in action:

behavior_demo.mp4

[Calinou]

Tested locally, it works as expected.

However, when you restart the editor, all instances are back to the highest number of instances you've ever defined.

This means that for example, if you create 2 instances (having 3 in total), fill their arguments, reduce the instance count to 1, save and restart the editor, you'll see all 3 instances again once you restart the editor. This applies even after using right-click > Clear All.

[ckaiser]

Had to add a new project metadata entry to keep the value, because before we were just relying on the size of the list's size for the instance count field, but since now we're keeping all the data it's not really possible to do this without storing the number somewhere.

[KoBeWi]

When you clear an item, its arguments will travel one below.

godot.windows.editor.dev.x86_64_xqAA8F61Pa.mp4

[ckaiser]

😅 silly mistake, fixed and rebased.

[KoBeWi]

Looks good.

[akien-mga]

I think it's an improvement, but I find it a bit weird that some information is kept but not visible to the user. And the overall UX of having to decrease the number in the SpinBox which then removes lines with important configuration was IMO always subpar.

How about instead doing something like this:

• Remove the SpinBox, and add a "Add Instance" button that adds a new entry to the tree (could be similar to the "Add Element" buttons in the Inspector for array/dictionary editors.
• Add a first column with a checkbox to enable the instance.
• Add a Delete icon and possibly a Move button, again similar to the array/dictionary editors.

So you can have e.g. 4 instances configured, and just check the ones you want to be active. This also makes it easier to have various presets for different configurations and test them separately without having to rewrite things in a specific order.

My only minor concern is that there's already two "[ ] Enabled" columns and having a 3rd one might not make clear at glance what each checkbox affects, but IMO that would still be a net improvement compared to both the current status, and the implementation in this PR.

[KoBeWi]

Instance counter makes it easier to tell how many instances will start.
Maybe we can keep the SpinBox, but set its max value to number of added instances. The "Add Instance" button could work.
This would also allow to implement deleting of instances, as currently you can only do Clear All to remove extra stored data.

[akien-mga]

This is fine for now but I still think we should improve the UX further as I suggested in a comment.

[akien-mga]

Thanks!

[ckaiser]

Oh that took me by surprise, I was already working on the improved version but I'll take my time and do a new PR 😄