Vendor helgoboss/helgobox (ReaLearn) as basis for custom UI fork

Stripped upstream git history; starting point for replacing the native
SWELL/Win32 mapping UI with something more suited to bulk editing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugin-reaper-realearn/swell-ui/src/win.rs

@@ -0,0 +1,29 @@
+use libloading::{Library, Symbol};
+use winapi::shared::minwindef::UINT;
+use winapi::shared::windef::HWND;
+
+/// Provides access to some Win32 API functions that are not available in older Windows versions.
+///
+/// This is better than eagerly linking to these functions because then the resulting binary
+/// wouldn't work *at all* in the older Windows versions, whereas with this approach, we can
+/// fall back to alternative logic or alternative values on a case-by-case basis.  
+pub struct DynamicWinApi {
+    user32_library: Library,
+}
+
+impl DynamicWinApi {
+    pub fn load() -> Self {
+        unsafe {
+            Self {
+                user32_library: Library::new("user32.dll").unwrap(),
+            }
+        }
+    }
+
+    /// Should be available from Windows 10 onwards.
+    pub fn get_dpi_for_window(&self) -> Option<Symbol<GetDpiForWindow>> {
+        unsafe { self.user32_library.get(b"GetDpiForWindow\0").ok() }
+    }
+}
+
+type GetDpiForWindow = extern "system" fn(hwnd: HWND) -> UINT;