Press Back Button twice to quit GnuCashAndroid

[JeanGarf]

Steps to reproduce the behaviour

1. On Accout Activity, clicking Back button once quit GnuCashAndroid

Expected behaviour

1. It would be nice to have the option to click Back button twice to avoid quitting unexpectedly
2. A General Preference allow to enable/disable this function

Actual behaviour

Software specifications

• GnuCash Android version: 2.4.0
• System Android version: 9.0
• Device type: Samsung Galaxy Note 8

[JeanGarf]

I'have corrected this issue in 21b6e9f, but I don't know yet how to give it to you