Visual offset options for plot canvases

[birdzix]

When multiple plot intents are plotted into the same canvas, it may be difficult to distinguish the different plots, especially if they are close in their values. Adding an interface to add a visual offset (e.g in y-direction) could be used to better view the data relationships

[birdzix]

Could add a new mixin class in xicam.gui.widgets.imageviewmixins that adds some interface for offsetting plots on the same image view canvas. Note, we do want to make sure we don't take up too much of the image view box space with a bunch of buttons, line-edits, etc. For example, add a line-edit where its submit() (when you hit the return key) will update the offset (offset 0 indicates no offset). This could be added into the self.ui.right_layout or self.ui.left_layout, or, could be added below the image view in self.ui.outer_layout(?).