[tepl] docs: start to write new roadmap

[Sébastien Wilmet <swilmet src gnome org>]

commit c86e6686721e1c2b7ce75d17377a6f6c064ae233
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Tue Apr 14 18:54:06 2020 +0200

    docs: start to write new roadmap

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+Tepl roadmap
+============
+
+This page contains the plans for major code changes we hope to get done in the
+future.
+
+See the [NEWS file](../NEWS) for a detailed history.
+
+Rework file metadata
+--------------------
+
+- Status: **in progress**
+- Target release: Tepl 5 (GNOME 3.38)
+
+Tasks:
+- Make TeplFileMetadata independent of TeplFile, to better isolate toolkit
+  features.
+- Replace the metadata manager with TeplMetadataStore, to get rid of the libxml2
+  dependency (at least in Tepl, the libxml2 is still used by GtkSourceView).
+- If possible, limit synchronous I/O to application shutdown only, do
+  asynchronous I/O the rest of the time (during application startup and normal
+  execution).
+- Write more unit tests.
+
+Rework file loading and saving toolkit
+--------------------------------------
+
+- Status: **todo**
+
+Tasks:
+- Use the [libicu](http://site.icu-project.org/) for character encoding
+  _auto-detection_, not
+  [uchardet](https://www.freedesktop.org/wiki/Software/uchardet/).
+- Use the libicu for character encoding _conversion_, not iconv.
+- Feature parity with the GtkSourceView file loading and saving API.
+
+High-level file loading and saving API: separate it from the core framework
+---------------------------------------------------------------------------
+
+- Status: **todo**
+
+The purpose is to have a more minimal core framework, and have the file loading
+and saving high-level API in a separate sub-namespace: TeplFls for instance.
+
+Continue high-level file loading and saving implementation
+----------------------------------------------------------
+
+- Status: **todo**
+
+All the errors would be handled by Tepl, showing TeplInfoBars etc.