avoid GList in empty!(c::GtkContainer)

[jwahlstrand]

Also provide useful errors in a couple of GtkGrid methods.

Fixes #669

[coveralls]

Pull Request Test Coverage Report for Build 3216260320

• -1 of 12 (91.67%) changed or added relevant lines in 2 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.4%) to 63.216%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/layout.jl	6	7	85.71%

Files with Coverage Reduction	New Missed Lines	%
src/container.jl	3	72.22%

Totals
Change from base Build 3118173225:	0.4%
Covered Lines:	1667
Relevant Lines:	2637

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3216265554

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 12 of 13 (92.31%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 64.321%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/layout.jl	6	7	85.71%

Totals
Change from base Build 3118173225:	0.3%
Covered Lines:	1682
Relevant Lines:	2615

---

💛 - Coveralls

[codecov]

Codecov Report

Base: 63.44% // Head: 63.76% // Increases project coverage by +0.31% 🎉

Coverage data is based on head (d5ca3dd) compared to base (7cd80f7).
Patch coverage: 92.30% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #671      +/-   ##
==========================================
+ Coverage   63.44%   63.76%   +0.31%     
==========================================
  Files          32       32              
  Lines        2637     2638       +1     
==========================================
+ Hits         1673     1682       +9     
+ Misses        964      956       -8     

Impacted Files	Coverage Δ
src/layout.jl	80.64% <85.71%> (+3.22%)	⬆️
src/container.jl	76.36% <100.00%> (+9.69%)	⬆️

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[tknopp]

Thanks!