Rework how libloot's version is exposed in the API

- Don't use a class, it doesn't add anything.
- Use constexpr for the individual version numbers
- Use a function to get the revision string, for consistency with the
  version string.

docs/api/reference.rst

@@ -4,6 +4,15 @@ API Reference
 
 .. contents::
 
+Constants
+=========
+
+.. doxygenvariable:: loot::LIBLOOT_VERSION_MAJOR
+
+.. doxygenvariable:: loot::LIBLOOT_VERSION_MINOR
+
+.. doxygenvariable:: loot::LIBLOOT_VERSION_PATCH
+
 Enumerations
 ============
 
@@ -30,6 +39,10 @@ Functions
 
 .. doxygenfunction:: loot::CreateGameHandle
 
+.. doxygenfunction:: loot::GetLiblootVersion
+
+.. doxygenfunction:: loot::GetLiblootRevision
+
 Interfaces
 ==========
 
@@ -60,9 +73,6 @@ Classes
 .. doxygenclass:: loot::Location
    :members:
 
-.. doxygenclass:: loot::LootVersion
-   :members:
-
 .. doxygenclass:: loot::MessageContent
    :members: