Persist in-use docs per project

[jeremypw]

Based off #1700

Fixes #967

• Saves and restores list of in use (open or hidden) documents for each project open in the sidebar
• Provision to also store cursor position for each document (to be implemented)
• Moves the responsibility for restoring project docs out of MainWindow and into FileView.
• Setting the active project determines which documents are actually opened.