[LibOverhaul 05] Use the prefix increment operator

[MartinBriza]

📒 Description

Because of the (perfectly logical) way how C++ works, there's a (very) slight peformance benefit of using the prefix increment operator (++it) compared to the postfix variant (it++) with iterators. cpplint complains about this so I wrote a patch to get rid of the warning.

🕶️ Types of changes

• New feature (non-breaking change which adds functionality)

🔎 Review hints

• Same as with most library overhaul PRs, warnings should disappear and the app should continue to compile and work the same on all platforms.

[NghiaTranUIT]

The last commit for this PR looks good to me 👍

[NghiaTranUIT]

This PR is branched off from https://github.com/toggl/toggldesktop/pull/3338, and this parent has an issues, which need to be address (https://github.com/toggl/toggldesktop/pull/3338#issuecomment-535377952), so I approve and merge it later when it's done.