gnu: icecat: Support Guix packaged extensions and native manifests.

* gnu/build/icecat-extension.scm: New file with a MAKE-ICECAT-EXTENSION
procedure that makes sure the add-on directory is a symlink, so that Icecat
can normalize it into a package store path.
* gnu/local.mk (dist_patch_DATA): Register it, as well as new patches.
* gnu/packages/browser-extensions.scm (ublock-origin)[properties]: Store the
add-on ID so that it is accessible in MAKE-ICECAT-EXTENSION.
[arguments]: Use the add-on ID as root directory.
(ublock-origin/icecat): New procedure.
* gnu/packages/gnuzilla.scm (icecat-minimal)[arguments]: Rewrite the unused
'apply-guix-specific-patches' phase so that it applies the following two
patches.
[native-search-paths]: New field.
* gnu/packages/patches/icecat-compare-paths.patch: New patch that compares
add-on paths (which are package store paths) to detect package changes.
* gnu/packages/patches/icecat-use-system-wide-dir.patch: New patch that
replaces "/usr/lib/mozilla" (the system-wide directory for extensions and
native manifests) with "$ICECAT_SYSTEM_DIR".

gnu/packages/patches/icecat-compare-paths.patch

@@ -0,0 +1,21 @@
+See comment in gnu/build/icecat-extension.scm.
+
+--- a/toolkit/mozapps/extensions/internal/XPIDatabase.jsm
++++ b/toolkit/mozapps/extensions/internal/XPIDatabase.jsm
+@@ -3452,6 +3452,7 @@ const XPIDatabaseReconcile = {
+     if (
+       newAddon ||
+       oldAddon.updateDate != xpiState.mtime ||
++      oldAddon.path != xpiState.path ||
+       (aUpdateCompatibility && this.isAppBundledLocation(installLocation))
+     ) {
+       newAddon = this.updateMetadata(
+@@ -3460,8 +3461,6 @@ const XPIDatabaseReconcile = {
+         xpiState,
+         newAddon
+       );
+-    } else if (oldAddon.path != xpiState.path) {
+-      newAddon = this.updatePath(installLocation, oldAddon, xpiState);
+     } else if (aUpdateCompatibility || aSchemaChange) {
+       newAddon = this.updateCompatibility(
+         installLocation,