fix(komorebi): account for border offset and width in layout

The layout should leave the space configured for the border, so that the
border always stays within the workspace bounds.

Border offset is cleaned up, as it is no longer a rect, but instead just
a fixed value.

The rect function for adjusting padding now takes a concrete value, as
the optional has no local meaning to the operation, being equivalent to
the default value.

A margin function is added to centralize the notion of increasing the
size of a rect by the given margin, the opposite of the padding
operation.

komorebi-core/src/arrangement.rs

@@ -189,7 +189,7 @@ impl Arrangement for DefaultLayout {
 
         dimensions
             .iter_mut()
-            .for_each(|l| l.add_padding(container_padding));
+            .for_each(|l| l.add_padding(container_padding.unwrap_or_default()));
 
         dimensions
     }
@@ -312,7 +312,7 @@ impl Arrangement for CustomLayout {
 
         dimensions
             .iter_mut()
-            .for_each(|l| l.add_padding(container_padding));
+            .for_each(|l| l.add_padding(container_padding.unwrap_or_default()));
 
         dimensions
     }