LibHTML+Browser: Show the number of pending resource loads

For now this is simply a counter+hook exposed by ResourceLoader and
shown in the Browser status bar.

This is not very nuanced, and it would be nice to expose more info so
we could eventually do something like a progress bar.

Applications/Browser/main.cpp

@@ -18,6 +18,7 @@
 #include <LibHTML/Layout/LayoutNode.h>
 #include <LibHTML/Parser/CSSParser.h>
 #include <LibHTML/Parser/HTMLParser.h>
+#include <LibHTML/ResourceLoader.h>
 #include <stdio.h>
 
 static const char* home_url = "file:///home/anon/www/welcome.html";
@@ -78,6 +79,14 @@ int main(int argc, char** argv)
 
     auto statusbar = GStatusBar::construct(widget);
 
+    ResourceLoader::the().on_load_counter_change = [&] {
+        if (ResourceLoader::the().pending_loads() == 0) {
+            statusbar->set_text("");
+            return;
+        }
+        statusbar->set_text(String::format("Loading (%d pending resources...)", ResourceLoader::the().pending_loads()));
+    };
+
     auto menubar = make<GMenuBar>();
 
     auto app_menu = make<GMenu>("Browser");

Libraries/LibHTML/ResourceLoader.cpp

@@ -35,7 +35,13 @@ void ResourceLoader::load(const URL& url, Function<void(const ByteBuffer&)> call
         request.set_url(url);
         request.set_method(CHttpRequest::Method::GET);
         auto job = request.schedule();
-        job->on_finish = [job, callback = move(callback)](bool success) {
+        ++m_pending_loads;
+        if (on_load_counter_change)
+            on_load_counter_change();
+        job->on_finish = [this, job, callback = move(callback)](bool success) {
+            --m_pending_loads;
+            if (on_load_counter_change)
+                on_load_counter_change();
             if (!success) {
                 dbg() << "HTTP job failed!";
                 ASSERT_NOT_REACHED();

Libraries/LibHTML/ResourceLoader.h

@@ -11,6 +11,12 @@ public:
 
     void load(const URL&, Function<void(const ByteBuffer&)>);
 
+    Function<void()> on_load_counter_change;
+
+    int pending_loads() const { return m_pending_loads; }
+
 private:
     ResourceLoader() {}
+
+    int m_pending_loads { 0 };
 };