fix(timer): return glib.SOURCE_CONTINUE instead true

[cnjhb]

The timer callback must return SOURCE_CONTINUE or SOURCE_REMOVE. Although these are essentially equivalent to true and false, this approach is more rigorous.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (cab3e81) to head (dcf1140).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4046   +/-   ##
=======================================
  Coverage   90.40%   90.41%           
=======================================
  Files         938      938           
  Lines       60250    60251    +1     
  Branches     1139     1139           
=======================================
+ Hits        54472    54478    +6     
+ Misses       5270     5267    -3     
+ Partials      508      506    -2     

Files with missing lines	Coverage Δ
lib/gears/timer.lua	95.29% <100.00%> (ø)

... and 2 files with indirect coverage changes

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