layout: Place items using simple loops

The C version of looping over buttons and other items was weakly typed, causing runtime errors, and also C doesn't know how to iterate in abstract, so it was full of callbacks with user-defined data. Moving this to Rust, iteration is made of simple loops, and compile-time type-checked, at the cost of some more verbose code.

eek/eek-xml-layout.c

@@ -929,7 +929,7 @@ eek_xml_layout_real_create_keyboard (EekLayout *self,
 
     for (uint i = 0; i < 4; i++) {
         if (views[i]) {
-            eek_layout_place_rows(keyboard, views[i]);
+            squeek_view_place_contents(views[i], keyboard);
         }
     }