libsysprof-gtk: add scaffolding for chart widget

This doesn't yet render things, but it starts to get the plumbing in place
so that we can create a custom row type and then consume the marks
catalog from SysprofDocument eventually.

src/libsysprof-gtk/tests/meson.build

@@ -13,6 +13,7 @@ libsysprof_gtk_testsuite_c_args = [
 
 libsysprof_gtk_testsuite = {
   'test-callgraph' : {'skip': true},
+  'test-mark-chart' : {'skip': true},
   'test-mark-table' : {'skip': true},
 }