UI/nvim_ui_attach(): add override option

Before now, Nvim always degrades UI capabilities to the lowest-common
denominator. For example, if any connected UI has `ext_messages=false`
then `ext_messages=true` requested by any other connected UI is ignored.

Now `nvim_ui_attach()` supports `override=true`, which flips the
behavior: if any UI requests an `ext_*` UI capability then the
capability is enabled (and the legacy behavior is disabled).

Legacy UIs will be broken while a `override=true` UI is connected, but
it's useful for debugging: you can type into the TUI and observe the UI
events from another connected (UI) client. And the legacy UI will
"recover" after the `override=true` UI disconnects.

Example using pynvim:

    >>> n.ui_attach(2048, 2048, rgb=True, override=True, ext_multigrid=True, ext_messages=True, ext_popupmenu=True)
    >>> while True: n.next_message();

src/nvim/ui.h

@@ -48,9 +48,11 @@ typedef int LineFlags;
 
 struct ui_t {
   bool rgb;
+  bool override;  ///< Force highest-requested UI capabilities.
   bool composed;
-  bool ui_ext[kUIExtCount];  ///< Externalized widgets
-  int width, height;
+  bool ui_ext[kUIExtCount];  ///< Externalized UI capabilities.
+  int width;
+  int height;
   void *data;
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS