libgpaste-applet: only display EmptyHistory if not already empty

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
libgpaste-applet: watch max-displayed-history-size changes

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
Merge branch 'gpaste-3.10'

* gpaste-3.10:
  libgpaste-applet: watch max-displayed-history-size changes
  libgpaste-applet: only display EmptyHistory if not already empty