gitk: fix trackpad scrolling for Tcl/Tk 8.7+

TIP 684 [1] introduced TouchpadScroll events in Tcl/Tk 8.7, separating
trackpad gestures from traditional MouseWheel events. This broke
trackpad scrolling in gitk where trackpads generate TouchpadScroll
events instead of MouseWheel events.

Fix that by adding TouchpadScroll event bindings for all scrollable
widgets following the TIP 684 specification. Implement a new
precisescrollval proc to handle the smaller delta values from
TouchpadScroll events, using appropriate scaling factors that seem
sensible on my MacBook.

Fixes https://github.com/j6t/gitk/issues/31.

[1]: https://core.tcl-lang.org/tips/doc/main/tip/684.md

Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>

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