libgpaste-daemon: add about

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
libgpaste-client: add about

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

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

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
gpaste-settings: use gpasteclient for about

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
gpasted: set dark theme

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