Asset System: "Mark Asset" & "Clear Asset" operators and UI integration

This makes it possible to turn data-blocks into assets and back into normal
data-blocks. A core design decision made for the asset system is that not every
data-block should be an asset, because not every data-block is made for reuse.
Users have to explicitly mark data-blocks as assets.

Exposes "Mark Asset" and "Clear Asset" in Outliner context menus (currently ID
Data submenu) and button context menus. We are still not too happy with the
names, they may change.

This uses the new context members to pass data-blocks to operators, added in
af008f5532 and 0c1d476923.

Part of the first Asset Browser milestone. Check the #asset_browser_milestone_1
project milestone on developer.blender.org.

Differential Revision: https://developer.blender.org/D9717

Reviewed by: Brecht Van Lommel

doc/doxygen/doxygen.source.h

@@ -121,6 +121,10 @@
  *  \ingroup editors
  */
 
+/** \defgroup edasset asset
+ *  \ingroup editors
+ */
+
 /** \defgroup edcurve curve
  *  \ingroup editors
  */