Need to enter an operand before using an operator

[Mihir Soni]

Expected behaviour:

- The user needs to enter an operand (e.g. 5) before an operator (e.g. +)

Current behaviour:

- The user can enter any operators (e.g. +, *, /) before entering any operand (e.g. 5). They'll just display on the screen without doing any function

Steps to reproduce:

1. Start the calculator app
2. Click on (+) or (*) or (-)

[Alan Pope 🍺🐧🐱 🦄 (popey)]

- should do something. All other calculators let you enter negative numbers by starting the calculator and typing "-3" for example.

Technically it's starting with 0 on the display but we don't show it. So typing "+3" from cold is fine too.

[Ubuntu Phone Apps Jenkins Bot (ubuntu-phone-apps-jenkins-bot)]

Fix committed into lp:ubuntu-calculator-app at revision 134, scheduled for release in ubuntu-calculator-app, milestone alpha-1