Improve custom sidebar button functionality

This fixes a couple issues, adds session persistence, creates a context menu option to restore deleted default buttons, and adds the ability to store the last used button to the OpenViews so it opens that automatically the next time the session is opened (but really, this needs to be moved to the session file).

Test:

• Drag folders from the sidebar tree (or any folder) onto the sidebar. This should create a new button. You can rename, change icon, set to show hidden, etc. Restart your session, and button should still be there (and all your customizations be the same).
• Add a few buttons, restart your session, all buttons should still be there.
• Remove one (or more) of the default sidebar buttons (Places, Root Folder, Home Folder, Fonts, etc), restart session, any removed buttons should not be present in the sidebar. Activate the sidebar context menu (not on a button though, on the sidebar itself) and use the option to "Restore all removed default buttons". All removed buttons should reappear. Restart the session and those buttons should still be present.
• Click on a different button than what showed up the first time in the sidebar. Restart session, your last selected button should be what is selected (and displayed) when the sidebar is opened again.