Add a tooltip to let user know how to rename images/canvases

[jamiefolsom]

Just one idea, but we need to help users learn how to do this, since renaming images/canvases only works via the right mouse button, as a left click is bound to drag behaviour of the window within the viewer grid. See #81

[akstuhl]

@jamiefolsom the drag handler blocking left-click to edit title, which @gremid correctly noted in #81, was fixed in #85. Does that resolve this issue, or would a tooltip / hover message for the title text (when resource is in edit mode) still be a good feature to add?