Update changelog for 0.3.0

Author: takev

CHANGELOG.md

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+Change log
+==========
+
+0.3.0 Fancy Frog
+----------------
+In this version we concentrated on making it easy for developers
+to install ttauri as a dependency for their own projects through vcpkg.
+
+Here are some of the important changes for this release:
+ * Removed the embedded dependencies and instead making them installable via vcpkg.
+ * Made a vcpkg ttauri-port so that ttauri can be used as a dependency.
+ * We've made several API usability improvements, such as:
+   - making the widget's coordinate system relative to the widget;
+   - now using high level geometric types such as vector, point and color in as many places as possible;
+   - replacing the hidden state in the draw context to explicit arguments in draw calls; and
+   - changing the cell-coordinate system when placing widget on a grid layout to Excel-like cell-coordinates.
+ * Fixed several bugs.
+ * Made several documentation improvements.
+ * Improved the pull-request work flow with continuous integration requirements before merging.