threaded image prefetch

[gdzhu]

WHY: On devices with slower I/O, loading images to QPixmap may take a long time (in my case the image directory is mapped to cloud storage). And since the timer is in its separate thread and fires signal to update the image with the configured timeout, adding image loading/processing time to the total images rotation duration negatively impacts the experience of the slide show by:

• causing images to be displayed with different durations, and/or
• when transition animation is added, potentially causing the animation to get cut off halfway.

WHAT:

• This PR moves image loading action from within the image update handler (MainWindow::setImage) to its own function in ImageSwitcher that can be parallelized to the main window image update thread.
• The image loading action is wired to a new imageUpdated signal, emitted when MainWindow::setImage finished executing. It fetches the next filename and loads the image into memory in the background while the QWidget is being repainted, so that when the timeout timer fires, the main window update don't need to wait for the image to load.

NOTE:

• C++ is not my expertise, and I'm not sure the threaded solution I chose ( a combination of QFuture, QFutureWatcher and QtConcurrent) is ideal.

• One caveat is that if the image is not successfully loaded when ImageSwitcher::updateImage is being invoked by the timer, the same (old) image will be displayed, effectively double the duration for that image and skipping the failed-to-load image (because the image update action will trigger another image load action), but with the original logic the same behavior would also be expected (but worse because of the added image processing time).
  One potential workaround is to block on ImageSwitcher::updateImage to update the image only when a new image is successfully loaded, but my lack of C++ knowledge makes it hard for me to have it implemented.
  The main drawing thread can block on image loading with QFutureWatcher.isRunning().

• This PR also includes changes from configurable transition time #42 and skip overlay processing when no overlay is specified #43, and those should be reviewed prior to this one.
  I was able to refactor the PR and remove unrelated changes.

[NautiluX]

couldn't we draw the image here and immediately render the next one in the same thread? That way we just had to make sure we don't run updateImage() again before the previous run is finished (when the render took longer than the timeout)

[gdzhu]

I think that would definitely be better. Tried this idea previously but could not get it to work, reason being (it seems to me) QWidget::update() will not draw the image until the function ImageSwitcher::updateImage() returns. Any suggestions or it's just me not familiar with QT?
Also tried offloading the rendering to a third thread, but that adds more coordination and I can open up another PR for it if desire.