leaflet: Add the over and under mode transition animations

This allows the mode transition animation to match the semantic of the
over and under child transitions.
7 jobs for wip/exalm/fix-230 in 8 minutes and 52 seconds (queued for 1 second)
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passed #193073
librem5
build-debian-clang

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passed #193071
librem5
build-debian-gcc

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passed #193072
librem5
build-debian-gcc-static

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passed #193074
librem5
build-fedora-gcc

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librem5
build-gtkdoc

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  Test
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librem5
unit-test

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  Package
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librem5
package-debian

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