IP-74-Parallel Image View

[mihane-ichinose]

• Added a top bar in image panel to toggle between sliders and parallel view.
• The image views in parallel view can adjust their widths based on width of the image panel.
• Fixed alignment of the horizontal slider to make it look nicer.
• Removed export parallel view.

[zhao-xuan]

First of all, the parallel view should be toggleable: there should be a button that can switch between parallel view and slider view.
Second, the size of the parallel view should be the same as the resulting image. If there is no space to put the two images together, it would be great to use panels: panels have a bar that can slide, which can save space
Finally, it won't be necessary to export the original image, so delete the "Export Current Parallel View"