qt: Modernize custom filtering

In `QSortFilterProxyModel`, `invalidateFilter()` is scheduled for
deprecation in Qt 6.13.
`beginFilterChange()` was introduced in Qt 6.9.
`endFilterChange()` was introduced in Qt 6.10.

qt: Modernize custom filtering

In `QSortFilterProxyModel`, `invalidateFilter()` is scheduled for
deprecation in Qt 6.13.
`beginFilterChange()` was introduced in Qt 6.9.
`endFilterChange()` was introduced in Qt 6.10.

qt: Fix compatibility with `-debuglogfile` command-line option

This change avoids hardcoding the "debug.log" filename, ensuring
compatibility with custom filenames provided via the `-debuglogfile`
command-line option.

qt: Avoid header circular dependency

qt: Avoid header circular dependency

qt, refactor: Move `FreespaceChecker` class into its own module

qt, refactor: Move `FreespaceChecker` class into its own module

qt, refactor: Move `FreespaceChecker` class into its own module

qt, refactor: Move `FreespaceChecker` class into its own module

qt, docs: Unify term "clipboard"