Improve rendering of formatting buttons

...main/java/de/kuschku/quasseldroid/ui/chat/input/Editor.kt

app/src/main/res/drawable-v21/bg_menuitem_rounded_dark.xml

app/src/main/res/drawable-v21/bg_menuitem_rounded_light.xml

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

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

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

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

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

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

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

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