GDK: Add cursor theme support to W32 backend

Load themed cursors from the same places they are loaded on freedesktop systems,
but use W32 API functions to do so (works for .cur/.ani cursors instead of X
cursors).

Refactor the code for cursor handling. Prefer loading cursors by name.

Do not load actual cursors when loading the theme. Find the files and remember
the arguments/calls for loading them instead. Keeping HCURSOR instance in the
hashmap would result in multiple GdkCursors using the same HCURSOR. Given that
we use DestroyCursor() to off them, this would cause problems (at the very
least - DestroyCursor() would fail).

Store GdkCursor instances in a cache. Update cached cursors when theme changes.

Recognize "system" theme as a special (and default) case. When it is set,
prefer system cursors and fall back to Adwaita cursors and (as a last resort)
built-in X cursors. Otherwise prefer theme cursors and fall back to system and
X cursors.

Force GTK to use "left_ptr" cursor when no cursor is set. Using NULL makes
it use the system default "arrow", which is not the intended behaviour when
a non-system theme is selected.

Ignore cursor size setting and query the OS for the required cursor size, as
Windows (almost) does not allow setting cursors of arbitrary size.

https://bugzilla.gnome.org/show_bug.cgi?id=749287

gdk/win32/gdkwin32display.h

@@ -50,6 +50,11 @@ typedef struct _GdkWin32DisplayClass GdkWin32DisplayClass;
 GDK_AVAILABLE_IN_ALL
 GType      gdk_win32_display_get_type            (void);
 
+GDK_AVAILABLE_IN_3_18
+void       gdk_win32_display_set_cursor_theme    (GdkDisplay  *display,
+                                                  const gchar *theme,
+                                                  gint         size);
+
 G_END_DECLS
 
 #endif /* __GDK_WIN32_DISPLAY_H__ */