Add option to create an untitled window on re-activation only

[ychin]

Discussed in https://github.com/macvim-dev/macvim/discussions/1482

^{Originally posted by ir0nh34d March 14, 2024} I have MacVim starting at login and I have set "Open Untitled Window" set to never so that it doesn't create a window at startup. I would like to be able to click on the MacVim icon in the dock and have it open a new window (as opposed to having to select "New Window" from the menu). So much like the option "only when MacVim launches" it would be nice to have an option "only when MacVim is re-activated".