GPasteHistory: init size when loading

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
GPasteHistory: reinit size on emptying

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
GPasteHistory: fix biggest election on removal

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>