Make editor and console windows dockable / make MDI mode a runtime option.

Window docking:

* Both editor and console are now dockable windows, default layout is the same as before. This allows both docking at all sides of the main window as well as optional undocking.
* Preferences has an option to prevent undocking so the windows can be moved around but not detached.

MDI mode:

* Available on all systems as runtime option in Preferences (was a compile time option)
* Trying to unify file open logic so the behavior is the same across "open", "open recent", "open example"

Drag&Drop behavior:
* ```*.stl, *.off, *.dxf``` pastes ```import(file.stl)``` at the text cursor position
* ```*.dat, *.png``` pastes ```surface(file.dat)``` at the text cursor position
* ```*.scad``` opens the file (in new window in case of MDI mode)