Fix PHP notices in boot, index and App

- Use defaults() with $_GET variables in boot
- Initialize $a before checking for object member in boot
- Initilialize the $_SESSION variable for backend mode in index
- Add member variable page_offset to App

src/App.php

@@ -37,6 +37,7 @@ class App {
 	public $query_string;
 	public $config;
 	public $page;
+	public $page_offset;
 	public $profile;
 	public $profile_uid;
 	public $user;