fix(focus_follows_mouse): don't reset g_window_manager.ffm_mode when it is called without a corresponding MENU_OPENED

[johnko]

i observed these logs when running yabai --verbose:

EVENT_HANDLER_WINDOW_TITLE_CHANGED: Terminal 46053
EVENT_HANDLER_APPLICATION_LAUNCHED: Google Calendar (42983) is not finished launching, subscribing to finishedLaunching changes
EVENT_HANDLER_WINDOW_TITLE_CHANGED: Terminal 46053
-[workspace_context observeValueForKeyPath:ofObject:change:context:]: Google Calendar (42983) finished launching
EVENT_HANDLER_APPLICATION_LAUNCHED: Google Calendar (42983)
EVENT_HANDLER_APPLICATION_FRONT_SWITCHED: Google Calendar (42983)
window_manager_create_and_add_window:46295 Google Calendar -  (AXWindow:AXStandardWindow:1)
EVENT_HANDLER_WINDOW_TITLE_CHANGED: Google Calendar 46295
EVENT_HANDLER_WINDOW_FOCUSED: Google Calendar 46295
EVENT_HANDLER_WINDOW_RESIZED: Google Calendar 46295
EVENT_HANDLER_WINDOW_MOVED:DEBOUNCED Google Calendar 46295
EVENT_HANDLER_WINDOW_TITLE_CHANGED: Google Calendar 46295
EVENT_HANDLER_MENU_CLOSED
EVENT_HANDLER_MENU_CLOSED

so i think MENU_CLOSED is being called without MENU_OPENED, which leaves ffm_value enum default to FFM_DISABLED.

• This patch just avoids setting g_window_manager.ffm_mode if is_menu_open was negative because that means it was probably called without a corresponding MENU_OPENED.

• (Alternatively, another fix could be saving ffm_value = g_window_manager.ffm_mode earlier than just in EVENT_HANDLER(MENU_OPENED))

fixes #2217