Update to use latest gotk3.

Author: linuxerwang

go.mod

@@ -1,5 +1,6 @@
 module github.com/linuxerwang/sourceview3
 
-go 1.14
+go 1.16
+
+require github.com/gotk3/gotk3 v0.0.0-20210514043925-3f44af595c5e
 
-require github.com/gotk3/gotk3 v0.4.0

go.sum

@@ -1,2 +1,6 @@
+github.com/gotk3/gotk3 v0.0.0-20210514043925-3f44af595c5e h1:XJMaCWPpLlLK8IqtPu671ur4g6dJu+J0hebNfK3t5bg=
+github.com/gotk3/gotk3 v0.0.0-20210514043925-3f44af595c5e/go.mod h1:/hqFpkNa9T3JgNAE2fLvCdov7c5bw//FHNZrZ3Uv9/Q=
 github.com/gotk3/gotk3 v0.4.0 h1:TIuhyQitGeRTxOQIV3AJlYtEWWJpC74JHwAIsxlH8MU=
 github.com/gotk3/gotk3 v0.4.0/go.mod h1:Eew3QBwAOBTrfFFDmsDE5wZWbcagBL1NUslj1GhRveo=
+github.com/gotk3/gotk3 v0.5.2 h1:jbSFvUNMfo3ImM6BWBAkNUxY5piqP3eTc1YFbYy9ecU=
+github.com/gotk3/gotk3 v0.5.2/go.mod h1:/hqFpkNa9T3JgNAE2fLvCdov7c5bw//FHNZrZ3Uv9/Q=