remove libav-tools dependency and fix gitignore

[qiyoulu]

For issue #176

[jonorthwash]

@qiyoulu, the version of the work in this PR doesn't seem to run. I get the following backtrace:

UltraTrace$ python3 -m ultratrace
WARN:   cannot import name 'ZoomFrame' from 'ultratrace.widgets' (/data/Documents/Research/PhonLab/qiyoulu/UltraTrace/ultratrace/widgets/__init__.py)
Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/data/Documents/Research/PhonLab/qiyoulu/UltraTrace/ultratrace/__main__.py", line 4, in <module>
    from . import modules
  File "/data/Documents/Research/PhonLab/qiyoulu/UltraTrace/ultratrace/modules/__init__.py", line 5, in <module>
    from .search import Search
  File "/data/Documents/Research/PhonLab/qiyoulu/UltraTrace/ultratrace/modules/search.py", line 1, in <module>
    from .textgrid import TextGrid
  File "/data/Documents/Research/PhonLab/qiyoulu/UltraTrace/ultratrace/modules/textgrid.py", line 4, in <module>
    from ..widgets import CanvasTooltip
ImportError: cannot import name 'CanvasTooltip' from 'ultratrace.widgets' (/data/Documents/Research/PhonLab/qiyoulu/UltraTrace/ultratrace/widgets/__init__.py)