Add options for selecting which platforms to support

[mrlacey]

Add a step to select which of the following to include support for in the generated app:

• Android
• iOS
• macOS
• Windows
• Tizen?

Must include at least one. - Need to check how to enforce this.

[mrlacey]

This is going to be a big change internally. Moving to the next release so can get more usable features out sooner

[mrlacey]

Moving to backlog while waiting to hear how much this is wanted by people using the tool