improve channel sidebar generally and add search

Before, since all the channel_listboxes of the entries are separate, there would be an issue that if you select channels from multiple guilds, it would weirdly look that they are all selected.

For this, the channel_listbox attribute is public, and created in init. Then the channel_sidebar listens to when a row is selected, in any of the guilds. And then sets that guild to be the exclusive one.