Add support for Premiere and Resolve proxies #1939

This pull request refactors and improves the proxy file detection and selection logic in the proxy manager. The main changes centralize and simplify how proxy files are located and chosen, prioritize Shotcut's own proxies over external ones, and add support for detecting external proxies in more folder structures.

Proxy file detection and selection improvements:

• Added ProxyManager::hashProxyFilePath to centralize logic for finding a proxy file in the project or global proxy folder, and updated all relevant code to use this function.


• Added ProxyManager::existingExternalProxy, which finds the first available external proxy (GoPro, DJI, or in Proxy/Proxies folders with flexible naming and extensions), and refactored code to use this function when Shotcut's own proxy is not present.


• Refactored proxy selection to always prefer Shotcut's own proxy if available, and only fall back to external proxies if not, ensuring more consistent proxy usage.

Code cleanup and modernization:

• Removed the now-obsolete ProxyManager::fileExists method and replaced its usage with the new centralized detection logic.


• Improved code readability and maintainability by consolidating proxy path logic, reducing duplicate code, and updating copyright years.

These changes make proxy handling more robust and extensible, and should reduce bugs related to proxy file detection and selection.