Resource manager improvements

[kirk23]

Please make it work like bitmap panel in old XAra.

1. You could drop a bitmap in  by drag'n'drop from any outside file manager , Dam soft  etc

2. you could drag a resource  from the  resource manager  over an embedded layer  in main  viewport and it replaces that specific embedded doc.  With Shift pressed  >>> all same embedded docs,  with Alt or something pressed  the new replaced image would be  keeping same dimensions ( width and height)   whatever dpi or actual pixels the original is

3.  when you save a doc it cleans unused resources or you can do it with a button.

4. make it something looking like assets panel