Make Gtk relocatable

[staticfloat]

Because Gtk copies some library names from the GTK3_jll module in a top-level directive, those values get baked in at precompile time. This could be fixed by using libgtk3 instead of libgtk everywhere, but we can also create a stateless binding by using the using Foo: a as b syntax, so this is the minimum change necessary.

[coveralls]

Pull Request Test Coverage Report for Build 4057989563

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

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.1%) to 69.086%

---

Totals
Change from base Build 3788169661:	2.1%
Covered Lines:	1875
Relevant Lines:	2714

---

💛 - Coveralls

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.45%. Comparing base (881fdce) to head (0bed2c0).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #687   +/-   ##
=======================================
  Coverage   68.45%   68.45%           
=======================================
  Files          32       32           
  Lines        2739     2739           
=======================================
  Hits         1875     1875           
  Misses        864      864           

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