Fully support right click menu "Check Out File" for Workspaces (i.e. "Open Folder")

Fixed the issue with right clicking on the editor tag or in the Solution Explorer for Workspaces (using "Open Folder" instead of loading a solution file) where "Check Out File" wasn't being handled properly. You should now be able to check out files in Workspaces by right clicking on the editor tag or by right clicking in the Solution Explorer menu.

There is a limitation in how the right click menu for the Solution Explorer works with Workspaces (when using "Open Folder") where if you check out a file from within Visual Studio. If you check out a file it will change the menu item "Check Out File" to be disabled (indicating the file is checked out), but if you then revert the file in P4V, the Solution Explorer right click menu will not change to enable the "Check Out File" item again (it will still be disabled). You can still check out the file by right clicking on the editor tab for that file instead of checking it out from the Solution Explorer. I was not able to find a good solution to fix this issue, but will keep looking into it to see if I can correct this limitation in the future.