When `tab_bar.show_pinned_tabs_in_separate_row` is enabled,
double-clicking the empty space on the unpinned tab row creates a new
tab, but double-clicking the empty space on the pinned tab row does
nothing.

Add the same `on_click` double-click handler to the pinned tab bar drop
target so both rows behave consistently.

Release Notes:

- Fixed double-clicking empty space in the pinned tab row not opening a
new tab when `show_pinned_tabs_in_separate_row` is enabled.

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Co-authored-by: Joseph T. Lyons <JosephTLyons@gmail.com>