Use remote desktop to make iPad/Android tablet/computer as a secondary monitor

As we know some developers are using a secondary monitor to boost their productivity.
More and more workers are working outside of the office like in a coffee shop and they don't have the luxury to connect to the desktop monitor.

It will be handy to use their iPad/Android/Chrome tablet on hand as a secondary monitor.

Technically its possible by creating a virtual display and streaming the X window to vnc. The issue is the setup effort is too steep for a normal user. Would like to see elementary OS user can toggle a button and start to connect their devices as the secondary monitor.

For example there's an application called Virtscreen that make your iPad/Android tablet/computer as a secondary monitor on Linux, but elementary OS can't run it due to gnome tray icon imcompatibility.
https://github.com/kbumsik/VirtScreen

This task including
1. host a vnc server locally (related to #3 ship a remote desktop app with elementary os)
[Ref 1](http://siallagan-open.blogspot.com/2015/06/install-and-configure-vnc-on-ubuntu-and.html) [Ref 2](https://www.cyberciti.biz/faq/install-and-configure-tigervnc-server-on-ubuntu-18-04/) [Ref 3 elementary os specific setting](https://unix.stackexchange.com/questions/138332/vnc-on-elementary-os)
2. use xrandr to create a virtual display
3. use x11vnc to stream x window to vnc
4. provide a reasonable UI to config it

Reference
- [Use a second laptop or headed computer as another monitor via vnc](https://gist.github.com/foxsan48/166fae536ceb2214f795670c8c4c8ee2)
- https://unix.stackexchange.com/questions/326234/setting-up-vnc-server-and-no-physical-display-ubuntu-16-04-lts