Refactoring goals:

• Remove all view references fromSidePaneComponent
• Instead of having an instance of SidePane in SidePaneComponent for updating the view, we should use binding
• Remove unnecessary abstraction in SidePane like (separating components and visibleComponents)
• Maybe refactor SidePaneComponent into view and ViewModel
• and finally, fix View menu for sidebar option can be out of sync with sidebar state  #8115

Edit: I ended up redesigning the whole component so don't take the above goals as a grant

Added classes

• SidePaneContainerView / SidePaneContainerViewModel: This is the replacement for SidePane, it has almost the same actions/methods (show, hide, toggle, moveUp, moveDown) but in private because they are not used from the outside, it has one public method sidePaneVisibleProperty(sidePaneType) in the view, it returns the visibility property for sidePaneType, it's public because it's the only property used from the outside (check menu).
• SidePaneHeaderView: This class represent the header view that used to be created using SidePaneComponent#getHeader, this class controls the position/visibility of a SidePaneView using actions received in the constructor (moveUpCommand, moveDownCommand, closeCommand).
• SidePaneView: This is a replacement for SidePaneComponent, it glues together the content of the pane with the header.
• GroupsSidePaneView: This is a specialized implementation of SidePaneView with 2 differences, an afterOpening method and a special header GroupsSidePaneHeaderView because groups pane uses an extra button for toggling intersection union, I didn't put afterOpening in the base implementation because the other sidepanes (webSearch and OpenOffice) don't use it.
• GroupsSidePaneHeaderView: This is a specialized implementation of SidePaneHeaderView with an extra button for toggling intersection union, it handles it internally.
• WebSearchPaneView: I just copy-pasted WebSearchPane#createContentPane into its own class.

Notes

• The visibility property of side panes is updated when visiblePanes list changes in SidePaneContainerViewModel.
• I gave up on afterOpening and beforeClosing methods in SidePaneComponent because they are only adding and removing panes to/from preferencesService.getSidePanePreferences().visiblePanes(), preferences visible panes are now updated from SidePaneContainerViewModel in show() and hide(), the only side pane that does something beside that, is GroupsSidePaneView that's why we added afterOpening to it.
• SidePaneType has the title, icon, and action of the side pane so it can be considered a model in the new design.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository

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