Add default implementation for WRegion:current

[raboof]

This will automatically implement it for WRootWin, where the active subwindow
will typically be a WScreen.

Follow-up on #275

[buhl]

EDIT: Ignore this. I was on the wrong branch 🌵

I just tested this branch and the change doesn't seem to work
In notionflux

lua> ioncore.current():screen_of():parent():current()
nil
lua> ioncore.current():screen_of():parent():current_scr()
WScreen: 0x5609f8e0e358
lua>

I tried putting back this line
a297424#diff-fc859448b8d4c58d47e4a68b962a268eR498

and it worked again.
so there is something that region_current_default doesn't do right when on WRootWin

[buhl]

Now on the right branch I did this in notionflux

lua> repl.inspect_object(ioncore.current():screen_of():parent())
+ WRootWin: 0x55624d56c128
 + WScreen: 0x55624d56c0d8
  + WGroupWS: 0x55624d60e0e8
   + WTiling: 0x55624d46a648
    + WFrame: 0x55624d46a718
     + WGroupCW: 0x55624d60d628
      + WClientWin: 0x55624d619fc8

The repl.inspect_object if one of @wilhelmy repl helper functions runs up and down the object tree.
WRootWin.current now returns a WScreen without calling rootwin_current_scr 👍

[buhl]

See #281 (comment)