Add gradient to source list items

[sssoleileraaa]

Description

In order to share the same background image, there needs to be a new widget to contain the source list and conversation view (view_holder to be precise) in the MainView. Once this is added, along with #512, the same background image will show up behind the source list and conversation view.

[eloquence]

Discussed in backlog review today; we'll still need to decide if we really need a gradient on the conversation view (view_holder), or if it will be sufficient simply to add it to the source list item.

[sssoleileraaa]

@ninavizz will add updated zeplin screens to this issue

[ninavizz]

Going over my own files on this topic, I now remember that I decided against including a gradient in the sourcelist highlight, when exploring options several weeks ago. Ultimately, while quite neat-o and aesthetically appealing, concern arose that it would seem too "weird" to users, and thus might stoke concern there could be a GPU or some other technical/rendering issue. Because, Qubes. As such... it is with great sadness, I close this issue.