[SelectiveMasking] Fix masks UI update

[troopa81]

Create a data structure on stack to avoid useless QTreeWidgetItem heap memory allocation which slows down considerably widget populate. That allows then to remove some flaky code used to avoid as much as possible masking widget update, causing refresh issues.

I tested it with the #34942 use case where masking widget populate was almost 3s. Since those modifications, populate is less than 50ms, so we can just update the masking widget every time it's useful.

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 02adff2)

[strk]

Looks good to me. I've left some nitpicking comments but I specially love the testcase !

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply bc107e5ecc2... test(MaskUI): add tests to check selective masking widget behavior
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Auto-merging tests/src/gui/CMakeLists.txt
CONFLICT (content): Merge conflict in tests/src/gui/CMakeLists.txt

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-57679-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick bc107e5ecc2cbd8c57d18d88cb5bbb78199c9d12,e6646ec4c47e56019e2d64373d9d575d8904476a,738295481b92364884f8f6fbf933e9cccb27cbb4,02adff26d4e7b5d622dd6e40a6354a411bc01264
# Push it to GitHub
git push --set-upstream origin backport-57679-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-57679-to-queued_ltr_backports.