feat: Option to show notification about mode switches triggered by input

[kanru]

Fixes #213

[codecov]

Codecov Report

Attention: Patch coverage is 32.35294% with 23 lines in your changes missing coverage. Please review.

Project coverage is 53.59%. Comparing base (c55f1cb) to head (335c17b).

Files	Patch %	Lines
src/ibus-chewing-engine.c	28.57%	20 Missing ⚠️
src/IBusChewingPreEdit.c	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage   53.99%   53.59%   -0.40%     
==========================================
  Files          17       17              
  Lines        2067     2101      +34     
==========================================
+ Hits         1116     1126      +10     
- Misses        951      975      +24     

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