Show dialog window to open Theia Hosted Instance in case browser blocks opening of a new tab

[olexii4]

Description

Hosted plugin manager for https://github.com/theia-demo-plugins/wiptheia doesn't start a plugin instance. And styles of select path window should be more readable.

Reproduction Steps

1. Open wiptheia and select path a plugin instance.
   
2. Start instanse.
   
3. Open a new page with the needed link. And there is no the checked plugin.
   In the case of env HOSTED_PLUGIN all is OK.

[mmorhun]

@olexii4 you can see the problem from the second screenshot. Your browser blocks popup. Allow them or open the url manually

[mmorhun]

So @olexii4 gives an idea to improve IDE behaviour in case popups is blocked. So we can add dialog with link to click to open a new tab with hosted instance.

[mmorhun]

Done for popup. Styles is outside this issue.