Problem:
_extui module name is confusing and should eventually end up in _core/.

Solution:
Move it there and name it ui2.

Problem:  Error copying over multi-line message span marks to expanded cmdline.
Solution: Only copy over the highlight marks as intended.

Problem:  UI2 does not implement the msg_show event msg_id parameter.
Solution: Store message IDs currently shown in the cmd/msg buffers.
          Set extmarks spanning the message which are used to replace
          a still visible message when a new message with the same ID
          is received.

Problem:  Dialog position unnecessarily adjusted for pum wildmenu.
Solution: Apply no offset if the pum is visible.

Problem:  The wildmenu is hidden behind the dialog window with "list" in 'wildmode'.
          Global ('laststatus' set to 3) statusline is hidden behind the
          pager window.
Solution: Check wildmenumode() to adjust the dialog position when necessary.
          Ensure pager is positioned above the global statusline with 'laststus' set to 3.

Problem:  Scheduled callbacks reference potentially outdated window
          handles e.g. after switching tabpage.
Solution: Use ext.wins which stores the updated window handles.

Problem:  Fast context for msg_show event inhibits vim.ui_attach from
          displaying a stream of messages from a single command.

Solution: Remove fast context from msg_show events emitted as a result
          of explicit API/command calls. The fast context was originally
          introduced to prevent issues with internal messages.

Problem:
Trying to match the search highlight groups to the Normal highlight for
the window can fail when the message highlighting contains a fg/bg that
the Normal highlight doesn't (like an error message in cmd will have
ErrorMsg highlight instead of MsgArea - which is Normal in cmd.)

Solution:
Link the search highlight groups to an empty group in 'winhighlight'
thus disabling them instead of overriding them with Normal/MsgArea/etc.

fix(ui2): hide search highlights in msg window.

Problem:  Search highlighting is shown in the msg (and dialog) window.
Solution: Hide search highlighting in all but the pager window.

Problem:  Resize events during startup may clear an active external
          cmdline, which is then not redrawn.
          UI2 VimResized autocommand does not work.
          UI2 message appearance may be altered by inherited window
          options. The message separator uses the wrong fillchar.

Solution: Unset cmdline_was_last_redrawn when clearing the screen, such
          that cmdline_show is re-emitted.
          Ensure set_pos function is called without arguments.
          Ensure such options are unset. Use 'fillchars'->msgsep.