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/doc.go

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+/*
+Package app provides a platform-independent interface to operating system
+functionality for running graphical user interfaces.
+
+*/
+package app