appdata: update screenshots data

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
libgpaste-core: fix assertion

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
libgpaste-applet: don't close on switch

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
libgpaste-applet: make switch usable on unity

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