Add support for "isEnabled"

[toddanglin]

To make DropDown consistent with other NativeScript UI elements, it would be great if there were support for a isEnabled property.

When isEnabled is false the DropDown should A) not open when tapped, and B) display the label in a disabled state. isEnabled should be true by default.

[lukeramsden]

It seems to work on Android, but just not iOS.

@PeterStaev Could we not just add a check for the isEnabled context and just return out of the function if it's false. Seems easy to me. I'd do it myself but I don't understand enough about how this TypeScript class stuff works.

[PeterStaev]

@lukeramsden , I will have to take a look at this, but it will be for the 3.1 version. Dont think just returning will do. You have to also close the popup. Also not sure about styles.

[lukeramsden]

@PeterStaev is there any sort of ETA for 3.1?

[lukeramsden]

Styles can be done with pseudo selectors NativeScript/NativeScript#50

[PeterStaev]

@lukeramsden , currently there is not ETA for 3.1.