Make file open and file save dialogs use GtkFileChooser

[Photon89]

Currently GtkFileSelection is used which is available in GTK2 as well but looks quite dusty.

[Photon89]

"Load object..." is now ported to GtkFileChooser via 2c0f2bc.
"Export SRS..." functionality is removed via bf1d0b9.
"Save snapshot..." is blocked by #2.