Implement context-sensitive warnings for outdated cores

...main/java/de/kuschku/quasseldroid/ui/chat/ChatActivity.kt

.../quasseldroid/ui/chat/buffers/BufferViewConfigFragment.kt

...n/java/de/kuschku/quasseldroid/util/helper/ThemeHelper.kt

...in/java/de/kuschku/quasseldroid/util/ui/WarningBarView.kt

app/src/main/res/drawable/ic_alert.xml

app/src/main/res/layout-land/layout_main.xml

app/src/main/res/layout-sw600dp-land/layout_main.xml

app/src/main/res/layout/fragment_chat_list.xml

app/src/main/res/layout/layout_main.xml

app/src/main/res/layout/widget_warning_bar.xml

app/src/main/res/values/strings.xml

app/src/main/res/values/styles_widgets.xml

...ava/de/kuschku/quasseldroid/viewmodel/QuasselViewModel.kt