window transparency / opacity feature

Nobody has claimed this yet.

Users with smaller screens (e.g., laptops) may benefit from the ability to control the background transparency view of the editor window, because a "see through" window would allow the user to copy/revise from other documents simultaneously without having to tab-switch to other windows.

This feature has been part of [iTerm2](https://github.com/gnachman/iTerm2) for many years, and is also present in MacOS native Terminal.app. It is also present in TextMate2 in versions 2.0.6 and below, but recent versions have broken the feature.

The idea is that the user can set the window's background view to a desired transparency / opacity, but the window will draw foreground objects like text, highlighting, or images at full. [Here's an SO post with suggestions in Swift, not super relevant but maybe a start since MacDown uses NSWindow?](https://stackoverflow.com/questions/34531118/how-can-i-create-a-window-with-transparent-background-with-swift-on-osx) Finally, here's a sreenshot of what it looks like with TextMate 2.0.6 on MacOS 10.13.6.

Start by examining the editor window's NSWindow usage and the linked Swift transparency suggestions. The feature is done when users can control the window's background opacity while foreground text, highlighting, and images remain fully opaque.

Written by the indexing model from the issue text.