support both syntaxes 'repeaterFields" and 'fieldContexts'

[gebeer]

Hi, please check. Now we can define repeater fields either way:

1. assoc array with fieldname => [fielddata]

   'repeaterFields' => [
   'foo' => ['columnWidth' => 50],
   ]

2. plain 'repeaterFields' array with fieldnames only + 'fieldContexts' assoc array with fieldname => [fielddata] to support copy/paste from RM Code in field edit screen

'repeaterFields' => [
  'foo',
],
'fieldContexts' => [
  'foo' => [...],
],