kitemlistcontroller: process forward/back buttons when double-clicking

kitemlistcontroller: process forward/back buttons when double-clicking

Tapping the forward or back mouse buttons quickly enough makes Dolphin
interpret the action as a double-click of the button in question and
handle it in mouseDoubleClickEvent() instead of its normal button
handler. This means that certain button presses might seem delayed or
"swallowed" when quickly navigating forwards or backwards through the
history.

Since a double-click of the forward or back button is currently
meaningless, fix this by emitting a normal mouseButtonPressed event for
those buttons in the double-click handler and skipping any further event
processing.

Co-authored-by: Felix Ernst <felixernst@zohomail.eu>
CCBUG: 485295