Improve aesthetics of notifications area

Replace PaletteMenuItem for Grid with a two labels
and one icon.

Other benefit of this approach is that we can now
control each label separetely.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Include notifications box properly

Instead of adding it to the secondary palette, simply
add it to the primary palette through the set_content
method.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Move clear option to the bottom

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Use Gtk.ScrolledWindow for notifications

Instead of letting the palette to grow infinitely,
use a Gtk.ScrolledWindow to limit the notifications
area to only 5 notifications.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Remove show_all

I don't like it.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Show notifications by clicking on icon

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Update corner notification PulsingIcon

As agreed during the design of this feature, the pulsing
icon for notifications should be seen at the top left
corner of the screen.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Update notification icon color

Use black and white colors, as presented in the original
design.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Draw badge according to design

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Remove unnecesary dependencies

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>