changed login behaviour
- start offlinemode if "Abbrechen" pressed or if the Userauthentication fails
- starts in onlinemode if the userauthentication was successfull
- added a static variable that indicates whether online or offlinemode is active
- added a static variable that inherits the logged-in username or null, if in offlinemode
if you are in offlinemode, you are now unable to change the tab
various code optimizations
- changed some Toasts from LONG to SHORT duration
- modified the timeout in HttpAction (3s -> 4s)