Wayland: Add support for file drop events

This adds support for file path drop events in text/uri-list format.

It is based on work by Pilzschaf in #2040.

Closes #2040

(cherry picked from commit 4cb36872a5)

src/wl_platform.h

@@ -171,6 +171,7 @@ typedef struct _GLFWofferWayland
 {
     struct wl_data_offer*       offer;
     GLFWbool                    text_plain_utf8;
+    GLFWbool                    text_uri_list;
 } _GLFWofferWayland;
 
 // Wayland-specific per-window data
@@ -250,6 +251,10 @@ typedef struct _GLFWlibraryWayland
     struct wl_data_offer*       selectionOffer;
     struct wl_data_source*      selectionSource;
 
+    struct wl_data_offer*       dragOffer;
+    _GLFWwindow*                dragFocus;
+    uint32_t                    dragSerial;
+
     int                         compositorVersion;
     int                         seatVersion;