gpaste-settings: modernize code

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
libgpaste-settings-ui: modernize

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
gpaste-settings: adapt to libgpaste-settings-ui changes

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
gnome-shell: port pref.js to new settings-ui

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
libgpaste-settings-ui: avoid double free

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