Show inbox if empty + handle empty folders

Fix #200 
This code makes sure the Inbox and the 'All Mail' folders are always visible in the navigation even if they are empty.

It also displays a 'No mail in this folder!' message when the folder is loaded. i used a slightly tweaked version of core's #emptycontent style for consistency.

@jancborchardt @DeepDiver1975 @Popoutdoor please review!