Edit in IDLE disabled in "Show more options"

[zooba]

When you right-click a .py file, you see the "Edit in IDLE" menu and it's enabled. If you then click "Show more options" the menu will be disabled.

However, if you shift-right click in the first place then the menu is enabled.

[zooba]

This is because the mutex added in #114 is per-menu, and we have multiple IExplorerCommand instances created in this situation.

We should probably only hold the mutex while reading runtimes, and if one gets removed/moved while the menu is up, the user will just get an error.