document-manager: sort files in document combo in ascending order.

Makes more sense than sorting them in descending order.
Add new quick-open plugin.
quick-open: Show opened documents at the top.
quick-open: Prettify copyright headers.
quick-open: Fixes
quick-open: Add separator between open files and project files.
quick-open: Don't crash when document without a file is added.
quick-open: Split out dialog into a QuickOpenDialog class.