clang-plugin: Fix handling of const types in the GSignal checker
clang-plugin: Improve error message cosmetics in the GVariant checker
tests: Check for all expected error lines rather than just one

Previously, the wrapper-compiler-errors script was assuming a test was
successful if it could see at least one of the expected error message
lines in the compiler output. Since several of those lines are
boilerplate, this hid legitimate test failures.

Fix it so that *all* expected lines are checked for (although there are
no checks which test that *only* those lines were output), and fix up a
few tests.
clang-plugin: Fix diagnostics to correctly use format placeholders
bugs: Add a bug about making it more obvious that Tartan is running
website: Add a troubleshooting section to the website
clang-plugin: Allow checkers to be individually disabled

By passing the --[enable|disable]-checker arguments to the plugin.