Issue#512: Navigate through conversations with hotkeys

[colefranz]

Contributor checklist:

• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest development branch
• My changes pass 100% of local tests
• My changes are ready to be shipped to users

Description

#512 #752
This was initially to be for issue 752 and noticed this as one of the priority fixes addressed in the issue and it didn't make sense for me to do all of the issues on one PR. This fix includes the following functionality:

• You can now switch between conversations using the ctrl/alt keys and up/down in unison.
• If you do not have a conversation selected it will select the first conversation

Tested on:

• Windows 10 64-bit.
• Ubuntu 17.10 64-bit Virtual Machine

New unit tests were also added to fully cover the feature.
Testing N/A to iOS or Android.
Manually testing included cycling through many conversations while modals were open or not and while focus was elsewhere. No issues.

[He-Ro]

This looks great, a big improvement for those that prefer the keyboard over the mouse!
I tested it and it works as advertised with one exception and I do not know if this is in the scope of this PR. If I initially open the app, then the new shortcuts do not work. I first have to click with my mouse anywhere inside the app or hit the tab key once. Then the shortcut works like a charm.
I am on Arch Linux and this happens when I open signal through my launcher as well as through a terminal. IMHO this should not hold this PR back, but might be considered a further improvement request.

[scottnonnenberg-signal]

Closing this for now, since it's been a while.