Avoid creating CaretBrowsing events on window focus

[Kestrer]

Currently, whenever the window is focused qutebrowser produces a JavaScript event. However, this makes it difficult to use the debugger with qutebrowser as every time I move my mouse back to the page everything gets paused and I'm not able to trigger my event.

Steps to reproduce:

1. Open the inspector on any page.
2. Go to the "Sources" tab.
3. Press the pause button (top-left of right panel).
4. Move mouse back on to page.
5. The script will pause.

[The-Compiler]

Reproduced - relevant code from caret.js:

CaretBrowsing.onWindowFocus = function() {
    CaretBrowsing.isWindowFocused = true;
    CaretBrowsing.updateIsCaretVisible();
};

CaretBrowsing.onWindowBlur = function() {
    CaretBrowsing.isWindowFocused = false;
    CaretBrowsing.updateIsCaretVisible();
};

// ...

CaretBrowsing.init = function() {
    CaretBrowsing.isWindowFocused = document.hasFocus();

    document.addEventListener("click", CaretBrowsing.onClick, false);
    window.addEventListener("focus", CaretBrowsing.onWindowFocus, false);
    window.addEventListener("blur", CaretBrowsing.onWindowBlur, false);
};

We should probably register/unregister those events when caret mode is entered/left instead.