Add an All tab to the dock that contains all slideshows

[jbwong05]

Is your feature request related to a problem? Please describe.
Some users would like the ability to control slideshows that are not on the preview or program scenes. See (#10)

Describe the solution you'd like
Add an All tab to the dock that would contain all text slideshows, not just the ones from the preview or program scenes

Describe alternatives you've considered
No other alternatives have been considered, but I am open to other possible solutions to this issue.

Additional context
Implementation would have to be carefully considered as having to enumerate all of the sources in order to refresh and update the dock could possibly add significant speed and performance overhead. Any input on this issue is welcome.