copen like window for task management

[aserowy]

To enable copen like windows. Multi Window Management should get implemented.

In the first step, we should seperate viewports from buffer to be able to show different 'buffer' within the same window.

Next, we should build a tree representation of view ports (named windows). Now, we can link buffer and window and add window movement.

In the last step we introduce the buffer for task managment with buffer type specific bindings to stop a task with e.g. 'dd'.

[aserowy]

next step should be an Enum like BufferType containing valid buffer options (text, file tree, copen etc. above current update and view.

It is important to move all file related updates into a new mod called filebuffer. The interface for filebuffer takes one cursor and one viewport and maps them to the underlying vp-cursor-buffer structure in FileWindow (which has to be renamed).