"Select Agent" Functionality

## Enhancement Request

When an application is started outside of a desktop agent environment, it will not connect to a DA.
In cases like this, the FDC3 connectivity won't work.

One option is that we open a popup requesting a URL for the desktop agent. So, the flow might be:

1. I have a Web Desktop Agent running in Tab 1
2. I (or something else) opens up an application that needs to connect to FDC3 in Tab 2.
3. Tab 2 wasn't opened / connected to Tab 1.
4. Tab 2 doesn't find the DA, but, having tried everything else pops up a window asking for a URL.
5. In the Web Desktop Agent, you go to a config screen and press a button to get a personalised URL for App2.
6. You paste the URL into Tab 2 and they handshake over DACP and work together.