fix bugs & refactor intro

Fixes,

• https://schluss.atlassian.net/browse/BETA-1651 - Download file button is again displayed too low
• https://schluss.atlassian.net/browse/BETA-1650 - Not able to open a stored UWV PDF file
• https://schluss.atlassian.net/browse/BETA-1635 - When doing nothing on registration screen, session expires and you need to login
• https://schluss.atlassian.net/browse/BETA-1648 - Timer active spam in log

Refactoring,

• https://schluss.atlassian.net/browse/BETA-1510
• At intro: make the intro_first.dart, intro_second.dart…etc into a generic intro_content widget and call the introContent(…) from intro.dart (see example at kluis.dart & kluis_content.dart)
• correct routing (it is now not using the right routing protocol, see: lib\ui\intro\intro.dart line 105 for example
• Define height & with in top of build method, so we have them here at initialization when needed, instead of accessing every time: