Switching channels in editor

Desired user workflow:

• Hovering "Add channel" reveals a sub-button "Add switching channel" (see #566)
• Clicking it opens a dialog with:
  • An input field for the switching channel name
  • A channel list for the switched channels -> similar to a mode's channel list, including the add channel button to either add existing or new channels
  • A dropdown to choose the trigger channel out of the other channels included in the current mode
  • As soon as a trigger channel has been selected: The trigger channel's capability table with a dropdown for each capability to choose the switched channel. → The default capability (i.e. that cap that is selected when the trigger channel is set to its default value) is somehow marked
  • A submit button
• After being submit, the switching channel is added the mode's channel list like a normal channel, but with the switched channels as sub-bullets.

It should also be possible to convert a channel in a mode's channel list into a switching channel: The dialog would open with the list of switched channels automatically including the original channel.

If the trigger channel has no default value when being selected, it should be automatically set to 0. If the trigger channel is edited later, the default value field should be required.

Start by locating the editor's mode and channel-list UI, especially the “Add channel” control and channel conversion flow. Trace how dialogs, trigger channels, capabilities, and default values are currently handled. Done means the specified switching-channel creation and conversion workflows work, including automatic default handling and the required validation when a trigger channel is edited.