Invoke-GuiCompletion should use TabExpansion2

[nightroman]

Invoke-GuiCompletion should use TabExpansion2, not [System.Management.Automation.CommandCompletion]::CompleteInput because TabExpansion2 opens doors for user extensions. The latter does not, it just calls the default not configurable method. I will submit the PR.

[nightroman]

BTW, the fix also resolves some noise, i.e. on calls to TabExpansion2 (global function) results do not contain GuiCompletion module variables and functions. On calls to CompleteInput made from the module, all the module stuff is "visible" and included in the results.