ui/app: add package documentation and document Main

Split Main into the common Main function and platform specific
main functions.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/os_wayland.go

@@ -152,7 +152,7 @@ var (
 	_XKB_MOD_NAME_SHIFT = []byte("Shift\x00")
 )
 
-func Main() {
+func main() {
 	<-mainDone
 }