Add libxkbregistry to query available RMLVO

This library is the replacement for clients parsing evdev.xml directly.
Instead, they should use the API here so that in the future we may even
be able to swap evdev.xml for a more suitable data format.

The library parses through evdev.xml (using libxml2) and - if requested -
through evdev.extras.xml as well. The merge approach is optimised for
the default case where we have a system-installed rules XML and another file in
$XDG_CONFIG_DIR that adds a few entries.  We load the system file first, then
append any custom ones to that. It's not possible to overwrite the MLVO list
provided by the system files - if you want to do that, get the change upstream.

XML validation is handled through the DTD itself which means we only need to
check for a nonempty name, everything else the DTD validation should complain
about.

The logging system is effectively identical to xkbcommon.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

1 files changed, 63 insertions, 0 deletions

diff --git a/doc/user-configuration.md b/doc/user-configuration.md
index 886950a..17e7179 100644
--- a/doc/user-configuration.md
+++ b/doc/user-configuration.md
@@ -154,3 +154,66 @@ xkb_symbols "baz" {
 
 With these in place, a user may select any layout/variant together with
 the "custom:foo" and/or "custom:baz" options.
+
+## Discoverable layouts
+
+**The below requires libxkbregistry as XKB lookup tool and does not work where
+clients parse the XML file directly**.
+
+The above sections apply only to the data files and require that the user knows
+about the existence of the new entries. To make custom entries discoverable by
+the configuration tools (e.g. the GNOME Control Center), the new entries must
+also be added to the XML file that is parsed by libxkbregistry. In most cases,
+this is the `evdev.xml` file in the rules directory. The example below shows the
+XML file that would add the custom layout and custom options as outlined above
+to the XKB registry:
+
+```
+$ cat $XDG_CONFIG_HOME/xkb/rules/evdev.xml
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE xkbConfigRegistry SYSTEM "xkb.dtd">
+<xkbConfigRegistry version="1.1">
+  <layoutList>
+    <layout>
+      <configItem>
+        <name>banana</name>
+        <shortDescription>ban</shortDescription>
+        <description>Banana</description>
+      </configItem>
+      <variantList>
+        <variant>
+          <configItem>
+            <name>orange</name>
+            <shortDescription>or</shortDescription>
+            <description>Orange (Banana)</description>
+          </variant>
+      </variantList>
+  </layoutList>
+  <optionList>
+    <group allowMultipleSelection="true">
+      <configItem>
+        <name>custom</name>
+        <description>Custom options</description>
+      </configItem>
+      <option>
+      <configItem>
+        <name>custom:foo</name>
+        <description>Map Tilde to nothing</description>
+      </configItem>
+      </option>
+      <option>
+      <configItem>
+        <name>custom:baz</name>
+        <description>Map Z to K</description>
+      </configItem>
+      </option>
+    </group>
+  </optionList>
+</xkbConfigRegistry>
+```
+
+The default behavior of libxkbregistry ensures that the new layout and options
+are added to the system-provided layouts and options.
+
+For details on the XML format, see DTD in `<datadir>/X11/xkb/rules/xkb.dtd`
+and the system-provided XML files in `<datadir>/X11/xkb/rulies/xkb.dtd`.