Bring back support for GTK versions pre 2.24

The introduction of GtkComboBoxText usage for selecting a map provider,
removed support for older versions of GTK on distros like Debian 6.0.4.

Parent class (GtkComboBox) methods have to be used instead, around
more pre-processor branching.

This patch also fixes a small memory leak when retrieving text from said
combo box.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>