add more tests, fix a few issues with GList, GtkGrid, GtkNotebook, GtkOverlay

[jwahlstrand]

Fixes a problem with finalizing GLists, appending GLists, and iterating over GSList. Also fixes a few issues in layout.jl found while adding tests.

[codecov]

Codecov Report

Merging #603 (591c7bf) into master (ce2673a) will increase coverage by 4.93%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #603      +/-   ##
==========================================
+ Coverage   51.86%   56.80%   +4.93%     
==========================================
  Files          32       32              
  Lines        2659     2669      +10     
==========================================
+ Hits         1379     1516     +137     
+ Misses       1280     1153     -127     

Impacted Files	Coverage Δ
src/GLib/glist.jl	84.35% <100.00%> (+16.12%)	⬆️
src/layout.jl	77.41% <100.00%> (+50.24%)	⬆️
src/GLib/GLib.jl	77.27% <0.00%> (-4.55%)	⬇️
src/base.jl	75.00% <0.00%> (+3.12%)	⬆️
src/toolbar.jl	90.00% <0.00%> (+10.00%)	⬆️
src/displays.jl	33.15% <0.00%> (+20.10%)	⬆️
src/container.jl	81.39% <0.00%> (+34.88%)	⬆️

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