Deprecate PageExtension and create ToolWindowExtension instead

[yuto-trd]

Is your feature request related to a problem? Please describe.

PageExtension is currently forced to display as a dialog.So it cannot be made to display as multiple windows.Also, unnecessary methods such as GetFilledIcon remain.

Describe the solution you'd like

Create a new ToolWindowExtension class so that the user can specify the option to display as a dialog or allow multiple windows.
Also, make it possible to retrieve the EditorContext since the structure has changed as follows since the docking layout was implemented.

Conventional

Application
  MainWindow
    EditPage
      EditorContext
      ...
    OutputPage
    ExtensionPage
    ...
    SettingsPage

Currently

Application
  MainWindow
    EditorHost
      EditorContext
      ...
  OutputDialog
  ExtensionDialog
  SettingsDialog

Describe alternatives you've considered

No response

Additional context

No response