Add widget page support for placeholder-plain

[sestr]

The widget for a signature could only be placed on the first page of a PDF document with the placeholder-plain package. However, it should also be possible to insert the widget on any other page. I made this possible by adding the widgetPage parameter to the configuration object of the plainAddPlaceholder method.

[coveralls]

coverage: 99.319% (-0.3%) from 99.636% when pulling 0f2052265137b034bb9f6fd2343b0b2cccda205b on sestr:feature/widget-page into 4a6a2a46a6a786f35d58a3e00c6acd13827445ee on vbuch:develop.