Add Install Mode

- Added $install_mode in Class App instead
- Extracted the install business logic to a new Class Install
- Replaced all $a->module == 'install' with $a->install_mode

src/App.php

@@ -52,6 +52,7 @@ class App
 	public $argv;
 	public $argc;
 	public $module;
+	public $install_mode = false;
 	public $pager;
 	public $strings;
 	public $basepath;