[player] Add optional backspace key to keypad with custom preset

projects/common/models/elements/element.ts

...b/input-groups/text-field-element-properties.component.ts

projects/editor/src/assets/i18n/de.json

...r/modules/key-input/components/keypad/keypad.component.ts

projects/player/modules/key-input/configs/key-layout.ts

...components/floating-keypad/floating-keypad.component.html

...onents/layouts/player-layout/player-layout.component.html