GPasteItem: use G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
GPasteHistory: use the max_memory setting

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
Merge branch '3.0'

* 3.0:
  GPasteHistory: use the max_memory setting