Seamless title bar

[nabsiddiqui]

Describe the solution you'd like

I have noticed that many applications are now utilizing a "seamless" title bar where the window decorations are embedded into the frame. I know there are already implementations on changing the title bar and also hiding it, but is there any desire to create a similar title bar for MacVim. I have provided an example from Typora below:

Seamless:

Classic:

[digicannon]

I'd like this.

[ychin]

Implemented in #888.