Split View Reformation

[atg]

• [x] Replace RBSplitView with a fresh split view implementation
• [x] Implement BigRational-based split ordering
• [ ] Implement pinned tabs
• [ ] Unify non-file views under one view controller superclass. So web preview and project find should have exactly the same window management behaviour, be embeddable, support multiple instances, jobs, etc.
• [ ] Implement segregated file/non-file split views in CHTabController (explained here). Non-file is web preview, project find, documentation, etc.
• [ ] Implement a non-file section at the bottom of active files in the splendid bar
• [ ] Fix the split view selector status bar menu thing... whatever it's called
• [ ] Rejoice