Async doesn't work right in the install manager

The async functions in the module installer still block the interface sometimes, and cannot be canceled. I need to rework how all of that functions and see if I can smooth out the installation process.

Key features:
* Pressing `Install` doesn't lock up the interface
* Pressing `install` shows a progress bar while downloading the module
* Multiple installs can be triggered in parallel and run in different threads
* An install can be canceled

There are some other actions like updating remote sources that should work better, but I'm not sure how yet.