feat(pages): add and configure widgets from their server schema - #3214

Part of #3099, on top of #3213 (grid canvas). The dialog that adds a widget to a grid or configures one, with a settings form generated from the widget types the server serves. #3215 opens it from the edit mode of a page.

Size: 587 lines: 488 code and 99 tests.

What changes

• The widget dialog asks for the type, a title, an icon from the named icon set, a description, and for a table or view widget one of the application's own tables and views.
• The settings form is generated from the schema of the type on GET /api/2/widget-types: text, long text, choices, a switch and colours. A new widget type needs a schema on the server and a renderer here, no new dialog. Required settings keep Add widget disabled until they're filled.
• Only types this frontend can show are offered. The server can know a type before the frontend renders it (the tabs type of feat(pages): let widgets sit in the tabs of a tabs widget #3212 until feat(pages): add a tabs widget whose tabs hold their own widgets #3217), and such a type is left out of the list.
• Configuring a widget keeps its icon, description and settings, and the type can't be changed.
• IconField picks a named icon for a form, with a button to clear it; feat(pages): show and edit grid pages of an application #3215 uses it for pages too.

How it was tested

• Cypress component tests: AddWidget.cy.js (only types it can show, the form from the server's schema with a required setting, icon and description kept when configuring, a data widget needs a table or view; the test of the offered types fails without the filter) and gridLayout.cy.js (the tables and views of the application)
• npm run lint, npm run stylelint and npm run build
• no PHP changes

🖼️ Screenshots

No screen yet: #3215 opens the dialog from a page, its pull request has the screenshots.

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔙 Backport requests are created or not needed: /backport to stableX.X
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI