Clicking an Image file in the project tree calls FileOpen command twice

Fixes https://github.com/adobe/brackets/issues/6001

Enable project manager to differentiate on documentSelectionFocusChange between 
a) the case where it needs to open the file before redrawing the tree or
b) go ahead and redraw because the custom viewer for the file entry is already open.

Essentially the fix does the same as the work around in DCHs did, that is check if we already display the file before calling the FILE_OPEN cmd.