Editor type plug-ins should be able to be rendered without a doc being present

[pascalwilbrink]

Currently only menu type plug-ins are supported to work without a doc being present.
This should also be the case for editor type plug-ins.

Acceptance criteria

• requiredDoc attribute should not only be present in menu type plugins but also editor type ones
• Editor type plugins should be able to be rendered without a doc being present.
  • From a given list of editor type plugins, only the ones with requiredDoc: false are visible when no doc is loaded and then if a doc is loaded all of them are visible.
• If no editor type plugins are rendered, don't show the mwc-tab-bar (CSS :empty)

[juancho0202]

After implementing this change in #1530, the tests for packages/openscd/test/unit/Plugging.test.ts stop working specifically at "add custom plugin dialog". The tests neither pass or fail just become unresponsive.

I noticed that this goes away after undoing the adding of requireDoc: true to all editor type plugins in packages/openscd/src/plugins.ts but this change is now required for all editor type plugins to tell apart the ones that should appear initially at the landing screen.