Fix the build for older glibmm versions.

Fix the build for older glibmm versions.

With older glibmm versions the following error occured:

  pv/toolbars/mainbar.cpp:228:6: error: no match for ‘operator!’ (operand
  type is ‘Glib::VariantContainerBase’)
    if (!gvar_dict) {
        ^

Work around the above issue by using .gobj() and not relying on
"Glib::VariantBase::operator BoolExpr() const", which was only
added in glibmm 2.36.

This fixes bug #548.

Tested on a Debian stable system with glibmm 2.32.1 and Boost 1.49.