[api,doc] Add stylesheed for doxygen

As suggested by @ctrlaltca:matrix.org the project logo now uses a
CSS to properly align its size.

docs/FreeRDP.css

@@ -0,0 +1,5 @@
+#projectlogo img {
+  border: 0px none;
+  height: 48px;
+  padding: 5px;
+}