Ability to hide a component if it's already installed

[icnocop]

The boostrapper should allow hiding a component if it's already installed.

This allows for the situation where the main application can run on either the Microsoft .NET Framework 3.5 or the Microsoft .NET Framework 4.0 for example.
Only one of the Microsoft .NET Frameworks is required.

The bootstrapper is configured with two required components:

1. Microsoft .NET Framework 3.5
2. Microsoft .NET Framework 4.0

Both components have the same installed checks:
.NET Framework 3.5 is installed or .NET Framework 4.0 is installed

The Microsoft .NET Framework 3.5 component has an operating system filter so that it is displayed/installed only on Windows XP, since the Microsoft .NET Framework 4.0 doesn't support all editions of Windows XP.
The Microsoft .NET Framework 4.0 component has an operating system filter so that it is displayed/installed on operating systems higher than Windows XP.

The bootstrapper is also configured to not allow running on operating systems less than Windows XP.

The issue with this configuration is that if the machine is running on Windows XP and only has the Microsoft .NET Framework 4.0 installed for example, the bootstrapper would indicate that the Microsoft .NET Framework 3.5 component is installed when it actually isn't.
I thought about changing the component to display just Microsoft .NET Framework, but then if the machine has the Microsoft .NET Framework 2.0 installed, the bootstrapper will indicate that the Microsoft .NET Framework is not installed and required and this may cause some confusion for end-users.

So the simplest solution I could come up with is to just hide the component if it's already installed (as determined by the installed checks).

I will be adding a new optional attribute called hide_component_if_installed on component elements which will have a default value of False.