plugins/shakecursor: Ensure that cursor is magnificated for at least certain amount of time

If the cursor is magnificated and it's slightly moved so the shake detector returns std::nullopt, the cursor scale will be set back to 1.0 as soon as possible. This is not ideal. Immediately resetting doesn't help with locating the cursor.

Also the cursor scale reset delay has been increased to two seconds to provide more time to see the cursor.