Dashboard: Add new visualization/row/library panel/pasted panel is now a dropdown menu

[polibb]

What is this feature?

Implement dropdown to add a new viz/library panel/row/pasted panel in the dashboard. Implemented in place of the current button in the top bar.
This PR:

• creates utils methods for creating new panel, new row, importing from library, and pasting a panel from clipboard
• creates a dropdown menu to be shown when clicking on Add icon in dashboard's nav bar
• removes always setting the panel-add icon to size xl and sets it to xl everywhere it was used until now
• removes automatic creation of new panel of type "add-panel" when calling getNewDashboardModelData when initialising a dashboard

Why do we need this feature?

Because it is better UX.

Who is this feature for?

Everyone

Which issue(s) does this PR fix?:

Fixes #64592

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.
• There are no known compatibility issues with older supported versions of Grafana, or plugins.
• It passes the Hosted Grafana feature readiness review for observability, scalability, performance, and security.

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[axelavargas]

Great job @polibb 🥳 , the code it's easy to read and well organized, I gave it a try locally and it works as expected, both with and without the feature flag.

P.S., I left a small nit, and I think we need some unit tests for the new components (but if time is scarce, we could always add the tests in a different PR).

[dprokop]

Did you figure out why ToolbarButton cannot be used here? Would love to understand that.

[polibb]

no 🥸

[dprokop]

A very similar type is defined in AddPanelWidget. Can we re-use that one?

[polibb]

the id of PanelPluginInfo in AddPanelWidget is of type number, whereas we are looking for a panel in panels here which are of type PanelPluginMeta which has the id of type string. (I suppose that's why const pluginInfo is set of type any, but that's not possible anymore - betterer is failing if I recreate the same method.)
The id is used as a holder for the panel's type in this search for the copied panel and then later for the creation of the pasted panel, so it suits it to be a string 😄.

I think we'll delete this AddPanelWidget alltogether when the feature flag emptyDashboardPage is removed?

Tbh, as a whole I was confused by this method - it's creating an array, but only ever fills it with 1 value; I'm not sure if pluginCopy.sort = -1; is necessary and why. The main reason I think any was used is because the defaults key is set onto the panelCopy (pluginCopy.defaults = copiedPanel;), but overall I think we can either return a tuple or this kind of an object of combined types which are just the panel type and a type that cares about defaults.

[natellium]

Not blocking the PR, but shouldn’t we localise these?