refactor: 150+ code quality changes pointed by Qodana

JetBrains' Qodana is a static code analyzer that provides some insights into code quality. I implemented more than 150 suggestions, mainly leaving about 45 that are essentially false positives like:

• public properties that are not used within the code, but only in the templates
• some Linq replacements for for-loops that makes the code harder to understand
• "Missing Type Specification" when using new() but for class properties that are well defined before