Add an option to control padding/spacing between items

Submitted by car..@..il.com

Assigned to Viktor Odintsev @ninetls

Link to original bug (#14517)

Description

Most gtk3 themes influence the spacing between applets in the panel but obviously not between indicators in the status notifier plugin.

The "square icons" options helps getting a more homogeneous panel to some extent, but sometimes the spacing between panels is way different than the spacing between indicators.

Please add a spacing/padding option for fine-tuning.