Cleanup in APIs to allow more useful things

Add SQL and HTML singletons instead of cluttering abstract classes

Add upper level ExportableObject to allow other than HTML

Move children handling into ExportableObject itself

Add beforeExecution and afterExecution signals

Add JsonDocument and NoOutputScript primitives

Fix code to use ExportableObject + mimetype for dynamic GET request

Fix code to use NoOutputScript for dynamic POST request

Add more error cases for GET and POST handling

Make Repeater more generic (no 'Html' prefix, and simple array as model)

Cleanup code (use QS/QB macros, replace foreach with iterators)