Simple particle picker

[brisvag]

Add a simple widget to create a new particle layer that allows manual picking, and can be quickly saved.

[codecov[bot]]

Codecov Report

Merging #137 (61d7285) into main (368815a) will increase coverage by 12.80%. The diff coverage is 88.37%.

@@             Coverage Diff             @@
##             main     #137       +/-   ##
===========================================
+ Coverage   72.47%   85.27%   +12.80%     
===========================================
  Files          10       10               
  Lines         287      326       +39     
===========================================
+ Hits          208      278       +70     
+ Misses         79       48       -31     

Impacted Files	Coverage Δ
blik/writer.py	0.00% <0.00%> (ø)
blik/widgets/main_widget.py	85.71% <88.88%> (+33.23%)	:arrow_up:
blik/_tests/test_reader.py	100.00% <100.00%> (ø)
blik/_tests/test_widget.py	100.00% <100.00%> (ø)
blik/reader.py	89.28% <100.00%> (+2.05%)	:arrow_up:
blik/widgets/filter.py	94.11% <100.00%> (ø)

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