assistant2: Move prompt editor item into dropdown menu (#27708)

This PR makes the plus icon button not a dropdown anymore, freeing it up
to be always the new thread action. In consequence, I'm moving all of
the other items into another dropdown, which now houses "new prompt
editor", history, and settings, all of which there are keybindings for.

<img
src="https://github.com/user-attachments/assets/1d0d43da-9447-4218-8b9b-e692c0b74f61"
width="700"/>
 
Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -629,7 +629,9 @@
     "bindings": {
       "ctrl-n": "assistant2::NewThread",
       "new": "assistant2::NewThread",
+      "ctrl-alt-n": "assistant2::NewPromptEditor",
       "ctrl-shift-h": "assistant2::OpenHistory",
+      "ctrl-alt-c": "assistant2::OpenConfiguration",
       "ctrl-i": "assistant2::ToggleProfileSelector",
       "ctrl-alt-/": "assistant::ToggleModelSelector",
       "ctrl-shift-a": "assistant2::ToggleContextPicker",