[gthumb-list] Latest changes

[Paolo Bacchilega <paolo bacchilega libero it>]

Hi,

I just pushed a series of 200 and more commits to master, so maybe it's 
a good idea to explain what changed.  There are two major changes I've 
done, one is about image loading and the other one is about the UI.

Images are only loaded at the requested size without loading the 
original image in a second step as happened in version 3.2.  The 
requested size is in general the size of the viewer, so an image of 
4000x4000 pixels will be loaded at less then half its size on a typical 
monitor, this will speed-up image loading of jpeg and raw images which 
are the formats that support scaled loading at the moment.

The other change should be obvious if you try the current master version 
and it's about the UI, this is a list of changes in no particular order:

* the menubar has been removed and a headerbar is used instead;
* images are activated with a single click (at the moment there is no
  option to use a double click);
* a dark theme is used by default;
* the image viewer uses an overview window to scroll the image instead
  of using the scrollbars;
* the image tools are organized on a grid instead of using a list;
* the statusbar has been simplified never showing both file list info
  and current image info at the same time.

And other little changes to improve usability.

- Paolo