Streamline full screen behaviours

Currently, some windows have a 'full screen' button (square icon).
Present:
- Blueprints (full with big margins & paddings)
- Statistics (completely fullscreen, no more top menu or anything). Pin button present while nothing else is visible. Research is also full-screen, but with menus overlayed.
- Trade: only vertically full.
Missing where it could be useful (non-exhaustive list):
- Population statistics
- Train manager
- Train depot
- Train station (very small window for Arrivals)
- Vehicle management
Suggestion:
At the very least, streamline it across the game. I propose the blueprints version of 'full screen', which keeps some contexts in the background.
As a proper in-depth solution, add resizable windows, from any corner, just drag and resize. Make the game remember the size of windows by type.