modes for iPad showGraph button(e.g. ⏈)

[igavrysh]

Modes to be supported for iPad only:

• SplitView, CalculatorView as a master view, GraphView as a detail view
• GraphView only
• CalculatorView only

Modes to be supported for iPhone only(currently is a default logic already implemented for both, iPad and iPhone):

• CalculatorView only
• GraphView only

Multiple tapping on showGraph button should cycle through these three states

State should be stored in UserDefaults and restore after app reopening