Make writeEtcDefaultKeyboard an option in keyboard.conf.

src/modules/keyboard/KeyboardPage.cpp

src/modules/keyboard/KeyboardPage.h

src/modules/keyboard/KeyboardViewStep.cpp

src/modules/keyboard/KeyboardViewStep.h

src/modules/keyboard/SetKeyboardLayoutJob.cpp

src/modules/keyboard/SetKeyboardLayoutJob.h

src/modules/keyboard/keyboard.conf