Rename "External Browser" to "External Web Browser" and put it in the correct tab

[manueljordan]

Hello

Since few previous releases is possible define an External Browser. It working through the Settings tab. I have two observations:

1. It is located in the Editor Settings tab - I think should be located in the Preview Settings. It because the Web Browser shows the preview at real time but externally of the AsciidocFX itself
2. Should be External Web Browser and not External Browser

I know they are not critical but have sense

Thanks for your understanding