Network status
Currently network problems are just ignored and no categories or images are shown.
We should inform the user that there is no network connection. after #21 is implemented it is should be highlighted, that only data from the cache is visible.
Maybe best would be to show a connection status in the navigation drawer: green: ok, connected yellow: synchronizing (upload or download, even for filling the cache ongoing) orange: network disabled due to setting, e. g. no network traffic without WLAN red: no network connection red: authentication failed
some aspects of this (how to enter different status) would require a separate ticket. This one here is for the general mechanisms to set and show it in the UI.
also linked to #161