Improved code, refactored it a bit, implemented full null safety

QuasselDroidNG/src/main/res/drawable-hdpi/ic_drawer.png

...selDroidNG/src/main/res/drawable-mdpi/drawer_shadow.9.png

QuasselDroidNG/src/main/res/drawable-mdpi/ic_drawer.png

...elDroidNG/src/main/res/drawable-xhdpi/drawer_shadow.9.png

QuasselDroidNG/src/main/res/drawable-xhdpi/ic_drawer.png

...lDroidNG/src/main/res/drawable-xxhdpi/drawer_shadow.9.png

QuasselDroidNG/src/main/res/drawable-xxhdpi/ic_drawer.png

QuasselDroidNG/src/main/res/layout/activity_core_setup.xml

QuasselDroidNG/src/main/res/layout/drawer.xml

...elDroidNG/src/main/res/layout/fragment_storagebackend.xml

...idNG/src/main/res/layout/fragment_storagebackend_list.xml

QuasselDroidNG/src/main/res/layout/header.xml

QuasselDroidNG/src/main/res/layout/login_dialog.xml

QuasselDroidNG/src/main/res/layout/slider_main.xml

QuasselDroidNG/src/main/res/menu/global.xml

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