New features: Elegantly handle closed DWM windows, DWM "fix" and "split" functions

This combines some of the functions I put together for dwm.vim.

DWM_Fix() will rearrange the windows, essentially stacking them and moving the current window to the main position.

DWM_Split() will perform a reasonable split... E.g., it will create a split directly below itself.  If it's the main window, it will move the split to the stack

There's a new autocommand (enabled by a variable) that will automatically check for "w:dwm" variables when a buffer is closed.  Those variables are set by DWM_New() and DWM_Split().  If the autocommand finds one, it will try to clean up the remaining windows.  It will also adjust the quickfix window so that it always opens along the bottom, since I would imagine most people prefer that.

I tried to avoid making this too opinionated, but let me know if you want things separated out or altered.