[GTKUI] Reduce status tab border to 1px
[GTKUI] Rework the sidebar layout

 * Changed variable names to be less confusing.
 * Flake8'd.
 * Move the 'count' to a separate render cell.
 * Reduced size of expander icon to make it less intrusive.
 * Enabled ellipsis on labels so count is still visible.
 * Used pango markup on cell labels and count.
 * No longer set a fixed colour to fix #1193.