Some drag'and'drop improvements.

First: disable d'n'd events by default; most apps don't need these at all, and
if an app doesn't explicitly handle these, each drop on the window will cause
a memory leak if the events are enabled. This follows the guidelines we have
for SDL_TEXTINPUT events already.

Second: when events are enabled or disabled, signal the video layer, as it
might be able to inform the OS, causing UI changes or optimizations (for
example, dropping a file icon on a Cocoa app that isn't accepting drops will
cause macOS to show a rejection animation instead of the drop operation just
vanishing into the ether, X11 might show a different cursor when dragging
onto an accepting window, etc).

Third: fill in the drop event details in the test library and enable the
events in testwm.c for making sure this all works as expected.

test/testwm2.c

@@ -146,6 +146,9 @@ main(int argc, char *argv[])
         quit(2);
     }
 
+    SDL_EventState(SDL_DROPFILE, SDL_ENABLE);
+    SDL_EventState(SDL_DROPTEXT, SDL_ENABLE);
+
     for (i = 0; i < state->num_windows; ++i) {
         SDL_Renderer *renderer = state->renderers[i];
         SDL_SetRenderDrawColor(renderer, 0xA0, 0xA0, 0xA0, 0xFF);