Detect external file modifications immediately, matching Notepad++ behavior

[PCCV]

Description:

Closes #558.

Currently, NotepadNext only detects that a file was modified by another program when the tab is activated or the window regains focus (checkFileForModification() called only from activateEditor() and focusIn()). This means changes made externally can go unnoticed for a long time if the user stays on the same tab with the window in the background.

This PR adds a centralized QFileSystemWatcher in EditorManager to detect external modifications immediately, without waiting for a tab switch or focus change — matching Notepad++'s behavior.

Changes

• EditorManager: single shared QFileSystemWatcher (rather than one per editor, to conserve file descriptors as discussed in Use QFileSystemWatcher for opened Editors #558). Watches/unwatches paths on editorCreated, closed, and renamed (the latter also covers Save As and New→File transitions). A 250ms debounce via QTimer::singleShot absorbs the double-signal behavior common to inotify-based watchers. New signal editorFileChangedOnDisk(ScintillaNext*).
• MainWindow: connects to the new signal, guarded by isActiveWindow() and editor == currentEditor() so the dialog only appears for the currently visible tab in the foreground window — same UX as before, just triggered sooner.
• MainWindow::changeEvent(): new override reacting to QEvent::ActivationChange to reliably catch the "switch back to NotepadNext without changing tabs" case. The existing applicationStateChanged-based path doesn't reliably fire in this scenario under X11/Cinnamon; changeEvent is Qt's own cross-platform activation notification and closes that gap. Left the existing mechanism in place since checkFileForModification() is a no-op when nothing changed.
• Reload confirmation dialog now warns explicitly when the current tab has unsaved changes, since immediate detection makes the unsaved-changes + external-modification conflict much easier to hit than before (previously it required a coincidental tab switch). Reloading still discards local changes; this PR only makes that risk visible rather than adding merge/backup logic, which is out of scope here.

Known limitation

QFileSystemWatcher stops watching a file if it's replaced via atomic rename (as some editors/tools do on save). A best-effort re-arm (addPath if the path dropped out of fileWatcher->files()) is included, but this hasn't been exhaustively tested against every external tool's save strategy.

Tested

• External modification while tab active / window focused
• External modification while tab active / window unfocused, then refocus without switching tabs
• External modification while tab active / window unfocused, then refocus via tab switch
• Rename, Save As
• Delete/recreate
• Unsaved local changes + external modification (dialog now warns before discarding)