Add a filter when setting the Top Level VI

[crossrulz]

A filter needs to be added to prevent controls, malleable VIs, global variables, etc. from being set as the Top Level VI.
The current proposal is to add an extension check to only allow *.vi and *.vit to be set as the Top Level VI. To prevent global variables, a check for a block diagram may also be added.

[crossrulz]

There is a VI Type property that can be used to distinguish between a "Standard VI" and a "Global" or a "Control" along with some other types. Unfortunately, a malleable VI is also listed as a "Standard VI". But a combination of the extension check and type check should do the job.