Simplify file modified handling

[samschott]

1. Unify handling of created and modified files. They already mostly share the same logic.
2. Dynamically update the event type based on whether the file already existed in our index. Fixes #850.

[codecov[bot]]

Codecov Report

Merging #855 (386c65d) into main (82e591f) will increase coverage by 0.03%. The diff coverage is 96.55%.

@@            Coverage Diff             @@
##             main     #855      +/-   ##
==========================================
+ Coverage   78.98%   79.02%   +0.03%     
==========================================
  Files          42       42              
  Lines        7481     7465      -16     
==========================================
- Hits         5909     5899      -10     
+ Misses       1572     1566       -6     

Flag	Coverage Δ
pytest	79.02% <96.55%> (+0.03%)	:arrow_up:

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Impacted Files	Coverage Δ
src/maestral/sync.py	86.80% <95.23%> (+0.27%)	:arrow_up:
src/maestral/models.py	96.47% <100.00%> (+0.02%)	:arrow_up:
src/maestral/client.py	83.76% <0.00%> (-0.15%)	:arrow_down:

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