feat: `with_has_shadow`, `has_shadow` and `set_has_shadow`

[michaelowens]

Is your feature request related to a problem? Please describe.
When using decorations: false the app removes not only the topbar & border, but also the shadow.

Describe the solution you'd like
I would like to see the shadow separated into it's own config property. So you could e.g. do:

"windows": [
  {
    "shadow": true,
    "decorations": false
  }
]

Which would result in a window without the top bar, without borders but with the OS shadow.

Describe alternatives you've considered
I tried building this in my app, but this comes with a lot of edge cases. You can't click through the shadow, you have to dynamically change the spacing/shadow when you are full screen, etc.

Additional context
This was experienced on Windows, I have not tested behavior on other OS'.

[amrbashir]

Windows:

I tried electron and found that:

• Shadows on electron are always ON no matter what whether the window is regular or frameless (without decorations), the only way to turn it OFF is to have a frameless transparent window.

Then I wrote a c++ app to experiment more with win32 apis

• Shadows can't be turned off for regular windows and they are off by default for frameless windows.
• We can add shadow to a frameless window whether it is transparent or not and turn it off too.

MacOS:

• From electron docs:

  On Mac, the native window shadow will not be shown on a transparent window.

• We already has set_has_shadow, with_has_shadow and has_shadow exposed in Mac specific platform traits.

Linux:

• Linux's shadows/transparency/blur is controlled by the compositor installed on the user system so we might not have much control over this at all.
• (Not tested) GTK3 has set_shadow_width method and GTK supports CSS so we can use box-shadow to add shadow to the window.
  Both of these methods might be able to bypass the compositor configuration but if they bypassed the compositor, they still can be overridden/controlled by whatever GTK theme is installed on the user system.
  • More info: https://askubuntu.com/a/1151079