Allow image cache to be clear

[Eroyi]

It appears that the image is cached based on the filename, which could potentially lead to issues:

1. In a scenario with multiple servers, images with the same name (but different content) could be shared across different servers (e.g., warning_red.png).
2. While testing the push demo, the image will be constantly modified to figure out the best way to present it. I have to change the name repeatedly to ensure that Bark obtains the correct image.

Issue 1 does have a way to work around it, 2 is much more imminent. I've tried reinstalling the app but had no luck, it would be great to have a cache clear button.