Allow binding mouse buttons to commands

[riverbl]

Allows specifying bindings that use mouseforward, mouseback and mousemiddle.

Does not allow specifying bindings that use the primary two mouse buttons.

Closes #2572.

I haven't added an entry to CHANGELOG.md, but can do if you'd like.

[codecov]

Codecov Report

Merging #2573 (5a92dda) into master (56a98b9) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2573      +/-   ##
==========================================
- Coverage   11.90%   11.90%   -0.01%     
==========================================
  Files         141      141              
  Lines       42703    42717      +14     
==========================================
  Hits         5084     5084              
- Misses      37619    37633      +14     

Files Changed	Coverage Δ
lapce-app/src/app.rs	0.00% <0.00%> (ø)
lapce-app/src/main_split.rs	0.00% <0.00%> (ø)
lapce-app/src/terminal/panel.rs	0.00% <0.00%> (ø)
lapce-app/src/window.rs	0.00% <0.00%> (ø)
lapce-app/src/window_tab.rs	0.00% <0.00%> (ø)

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