PluginSpec: Add a "LongDescription" to plugin meta data

The EmacsKeys plugin had a long description with details on what it
does, which in principle is a good thing, but shouldn't be shown with
e.g. the -version command line argument.

Split the description in a short "Description" (name unchanged for
compatibility), and a possibly longer "LongDescription", using only the
first one for -version, and showing both in the plugin details.

Adapt the EmacsKeys plugin meta data.

Fixes: QTCREATORBUG-17312
Change-Id: I4a4abf51e5e19b71ee73edb14c6a897fbceaf916
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/emacskeys/EmacsKeys.json.in

@@ -13,7 +13,8 @@
                   \"\",
                   \"Alternatively, this plugin may be used under the terms of the GNU General Public License version 3 as published by the Free Software Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT included in the packaging of this plugin. Please review the following information to ensure the GNU General Public License requirements will be met: https://www.gnu.org/licenses/gpl-3.0.html.\"
     ],
-    \"Description\" : [
+    \"Description\" : \"Provides additional actions that typical Emacs users would expect.\",
+    \"LongDescription\" : [
         \"The main idea behind this plugin is to provide additional actions a typical emacs user would expect. It doesn\'t claim to provide full emacs emulation. The following actions are available:\",
         \" - Movement [C-f, C-b, C-n, C-p, M-f, M-b, C-a, C-e, M-<, M->]\",
         \" - Mark-based selection [C-SPC, C-x C-x]\",