Adopt shorter identifiers

Also, slightly reformat the source code according to clang-format.

fiv-browser.h

@@ -0,0 +1,28 @@
+//
+// fiv-browser.h: fast image viewer - filesystem browser widget
+//
+// Copyright (c) 2021, Přemysl Eric Janouch <p@janouch.name>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+//
+
+#pragma once
+
+#include <gtk/gtk.h>
+
+#define FIV_TYPE_BROWSER (fiv_browser_get_type())
+G_DECLARE_FINAL_TYPE(FivBrowser, fiv_browser, FIV, BROWSER, GtkWidget)
+
+typedef gboolean (*FivBrowserFilterCallback) (const char *);
+
+void fiv_browser_load(
+	FivBrowser *self, FivBrowserFilterCallback cb, const char *path);