Important changes in the last commit that may break things (Episode 2)

[Benoît Minisini <benoit.minisini@xxxxxxxxxxxxxxxx>]

Hi,

I'm continuing implementing global style properties.

Now styled borders of Panel or ScrollArea (and all its children) 
automatically merge when they are contiguous, provided that they are 
arranged by their parent the same way, vertically or horizontally.

I have added a new 'Style.BorderFlat' boolean property. When set, the 
backgrounds are filled with a dark transparent color (or light on dark 
mode), and borders are removed.

But if two neighbours merge their borders, a one pixel separation is 
kept in that case.

I added an option in the IDE to toggle this property, so you can see by 
yourself how the GUI change.

The ProgressBar now follows the new styled borders.

The selection of GridView and its children (ComboBox, ListBox, 
TreeView...) is now flat, with only a thin border when the 
'Style.BorderFlat' is not set.

IconView selection drawing should be remade, it's not clean anymore.

I will update the existing controls progressively.

The next big step would be creating a new component for adding GTK+ 
Adwaita-like controls. With all that, we will be able to make Gnomish 
GUI (berk) in a way that will be compatible with Qt, and that will be 
adjustable by the user thanks to the 'Style.BorderShape' and 
'Style.BorderFlat' properties.

Any comment and ideas are welcome.

Regards,

--
Benoît Minisini.