Fix time position error if next file is shorter than current one
    
    When setting the new range, a "value-changed" signal is emitted,
    thus triggering the slider_cb which then triggers the audio_seek() and
    consequently invalidating our seek to 0.0.
Refactor wait_for_statechange into its own function

Might be needed more often in the future.
Allow audio_seek() to skip intermediate samples
[Makefile] Rebuild all .c files on any header change
Fix segfault due to uninitialised pointer