Add a standalone app mode

[Dylan McCall]

For some desktop integration work in Endless OS, it would be useful to add a "standalone" channel specific mode where the app tries to pin Kolibri to a specific channel. In a channel specific application, the window title should be set to the channel's name, and links within the channel should open in the same window while links to channels or other parts of Kolibri should be treated like external links, opening in a new window.