Don't use relative imports like `../foo.h` as those depend on the
relative position of both files. Rather than that use imports relative
to the `src` directory, which explicitly is part of the include
directories.
* built timer app
* Style improvements
* making sure buttons stay hidden when the app is reopened and reappear after the timer runs out
* more sensible calculations of time deltas. eliminated that mysterious scaling factor
* changing the timer icon
