I added several keyboard shortcuts so users can switch between the main
views of "Home", "DMs", "Search", and "Notifications" with the shortcuts
"CMD+1", "CMD+2" etc.. Those shortcuts made the most sense to me but
they're definitely up for discussion.
Added a new keyboard shortcut to creat a new post. Unfortunately it had
to be "SHIFT+CMD+N" because "CMD+N" already created a new window.
Closes: #163
Changelog-Added: Cmd-{1,2,3,4} to switch between tabs on MacOS
Changelog-Added: Shift-Cmd-N to create a post on MacOS
