tpdfv-ctl: Added controller module used to manage multiple GtkTPDFV widgets.

tpdfv-ctl: Added controller module used to manage multiple GtkTPDFV widgets.

The controller includes one "main" and an arbitrary number of "slave" widgets.
Each slave widget has a factor and an offset assigned to it. Those numbers
define how the current page of the slave relates to that of the master. Thus,
it might be used, for example, to specify a slave widget used to display a
preview of the next page.

The controller provides a subset of the functions provided by the GtkTPDFV
widget and forwards the calls to the "main" and "slave" widgets according to
the slave settings.

To ease integration, those functions either accept a controller object or a
GtkTPDFV widget as an argument and, in the latter case, directly forward the
call to the specified widget only.