Make MergeSide public, handle positions in MergeTagMaps differently & fix some bugs

[AndreasSko]

• ♻️ Make MergeSide public, as we are going to need it outside of the merger package to handle merceConflicts.
• :bug: Handle positions in MergeTagMaps differently, as the position is defined globally per Tag(!), not per PlaylistItemID/LocationID/NoteID (different to my first expectation 🤦).
• 🐛 UpdateIDs function not handling nil properly: If left and/or right were set to nil, the function would panic. Updated tests to detect similar errors in the future.
• 🐛 Remove Favorite tag from fresh DB: The Favorite tag is pre-included with a fresh JW-Library, however it should not be included in the fresh DB, as the merge process expects a empty table (so it can start at Tag ID 1). This solves a UNIQUE constraint error when exporting.

[coveralls]

Coverage decreased (-0.1%) to 79.459% when pulling c66b50421444550c61aa6243c29dc8404cd0b48f on bugfixes into 6ce4afaff3b3cfffdace9e01e3ee3d162c7b3d3f on master.

[codecov[bot]]

Codecov Report

Merging #8 into master will decrease coverage by 0.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   76.45%   76.30%   -0.15%     
==========================================
  Files          21       21              
  Lines         981      975       -6     
==========================================
- Hits          750      744       -6     
  Misses        175      175              
  Partials       56       56              

Impacted Files	Coverage Δ
model/Tag.go	96.87% <ø> (-0.19%)	:arrow_down:
merger/BookmarkMerger.go	100.00% <100.00%> (ø)
merger/IDChanges.go	100.00% <100.00%> (ø)
merger/LocationMerger.go	100.00% <100.00%> (ø)
merger/Merger.go	94.49% <100.00%> (ø)
merger/NoteMerger.go	100.00% <100.00%> (ø)
merger/TagMapMerger.go	100.00% <100.00%> (ø)
merger/TagMerger.go	100.00% <100.00%> (ø)
merger/UserMarkBlockRangeMerger.go	97.10% <100.00%> (ø)
model/binData.go	16.93% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 6ce4afa...c66b504. Read the comment docs.