Coding style, doxygen and clean-ups

Pushed random clean-ups to 1578300-kernel-coding-style.

Pushed some more.

Add TODOs

Pushed some more changes reacting to reviews:
Revert an accidental change to make error_test module hidden again:

--- a/core/modules/system/tests/modules/error_test/error_test.info
+++ b/core/modules/system/tests/modules/error_test/error_test.info
@@ -3,4 +3,4 @@ description = "Support module for error and exception testing."
 package = Testing
 version = VERSION
 core = 8.x
-hidden = FALSE
+hidden = TRUE

Revert an unnescessary change to the RewriteRule:

--- a/.htaccess
+++ b/.htaccess
@@ -108,7 +108,7 @@ DirectoryIndex index.php index.html index.htm
   RewriteCond %{REQUEST_FILENAME} !-f
   RewriteCond %{REQUEST_FILENAME} !-d
   RewriteCond %{REQUEST_URI} !=/favicon.ico
-  RewriteRule ^(.*)$ index.php [L]
+  RewriteRule ^ index.php [L]
 
   # Rules to correctly serve gzip compressed CSS and JS files.
   # Requires both mod_rewrite and mod_headers to be enabled.

:)

Pushed some more clean-ups, based on the review. Ok to re-use this generic clean-up issue?

:D