Provide for Android and iOS application icons.

Refine the elements.json specification format so that the source and target
files can have different names.

Rename after_prepare script icons.js to 10-icons.js and customize.js to
20-customize.js, for lexical sorting such that icons.js is executed first.

Add a batch of sample icons to test the arrangement, in 'brand_example'
directory beside custom/brand. To test, copy the contents from
brand_example to brand.