Dispose random view selectors

[zayenz]

Summary

• Add notice() and dispose() to the random view selector so its Rnd handle is destroyed when branchers are discarded.
• Mirror the existing random value selector resource-management pattern.
• Add a changelog entry thanking @k0stjap.

Fixes #62.

Validation

• cmake -S . -B /tmp/gecode-int-var-rnd-dispose-build -DCMAKE_BUILD_TYPE=Debug -DGECODE_ENABLE_EXAMPLES=OFF -DGECODE_ENABLE_GIST=OFF -DGECODE_ENABLE_QT=OFF
• cmake --build /tmp/gecode-int-var-rnd-dispose-build --target gecode-test -j 8
• /tmp/gecode-int-var-rnd-dispose-build/bin/gecode-test -iter 1 -test ^Int::Branch::Dense::3
• /tmp/gecode-int-var-rnd-dispose-build/bin/gecode-test -iter 1 -test ^Int::Branch::Sparse::3
• git diff --check

Note: I also started the full Int::Branch family, observed passing progress, and stopped it because it was too broad for this resource-management fix.