isInFocus() can utilize the platform-dependent GetCurrentWindowTitle() instead of re-implementing the same functionality.
Tested on Linux and macOS without issues
Direct matching before regex has the potential to interfere with some usecases. For example, if a user wants to enter regex syntax such as `shell[1]`, a direct match would return a window matching `shell[1]` exactly instead of `shell1`. Granted, the odds of the two conflicting each other are slim, but the core logic shouldn't prevent a user from utilizing regex to its full potential. One can always escape reserved characters if they want to match it directly. I have also added a warning and link to https://regexr.com in the UI should this be accepted.
The rest of what has changed is simply to align with the executable switcher's matching, namely the use of `QRegularExpression` to drop the need for a try/catch block and moving the check for fullscreen to the if statement in order to remove the possibility of a race condition.
