Use Chrome file dialogs on all platforms and runtimes (fixes issue #3314)

All file dialogs irrespective of source, platform and runtime will now be
routed through CefFileDialogManager and trigger CefDialogHandler callbacks
(see issue #3293).

Adds Chrome runtime support for CefBrowserHost::RunFileDialog and
CefDialogHandler callbacks.

Adds Alloy runtime support for internal GTK file and print dialogs on Linux
subject to the following limitations:

1. Internal GTK implementation:
   - Cannot be used with multi-threaded-message-loop because Chromium's
     internal GTK implementation is not thread-safe (does not use GDK threads).
   - Dialogs will not be modal to application windows when used with off-screen
     rendering due to lack of access to the client's top-level GtkWindow.
2. Cefclient CefDialogHandler implementation:
   - Cannot be used with Views because it requires a top-level GtkWindow.

Due to the above limitations no dialog implementation is currently provided for
Views + multi-threaded-message-loop on Linux. In cases where both
implementations are supported the cefclient version is now behind an optional
`--use-client-dialogs` command-line flag.

Expressly forbids multiple simultaneous file dialogs with the internal platform
implementation which uses modal dialogs. CefDialogHandler will still be notified
and can optionally handle each request without a modal dialog (see issue #3154).

Removes some RunFileDialog parameters that are not supported by the Chrome file
dialog implementation (selected_accept_filter parameter, cef_file_dialog_mode_t
overwrite/read-only flags).

libcef_dll/ctocpp/dialog_handler_ctocpp.cc

@@ -9,7 +9,7 @@
 // implementations. See the translator.README.txt file in the tools directory
 // for more information.
 //
-// $hash=cac933371fc47c864230d4537ef1da76cd7e4d8e$
+// $hash=336c57e3c57a1a5bf8fa4a39aee8f204788ccddb$
 //
 
 #include "libcef_dll/ctocpp/dialog_handler_ctocpp.h"
@@ -27,7 +27,6 @@ bool CefDialogHandlerCToCpp::OnFileDialog(
     const CefString& title,
     const CefString& default_file_path,
     const std::vector<CefString>& accept_filters,
-    int selected_accept_filter,
     CefRefPtr<CefFileDialogCallback> callback) {
   shutdown_checker::AssertNotShutdown();
 
@@ -41,10 +40,6 @@ bool CefDialogHandlerCToCpp::OnFileDialog(
   DCHECK(browser.get());
   if (!browser.get())
     return false;
-  // Verify param: selected_accept_filter; type: simple_byval
-  DCHECK_GE(selected_accept_filter, 0);
-  if (selected_accept_filter < 0)
-    return false;
   // Verify param: callback; type: refptr_diff
   DCHECK(callback.get());
   if (!callback.get())
@@ -60,7 +55,7 @@ bool CefDialogHandlerCToCpp::OnFileDialog(
   // Execute
   int _retval = _struct->on_file_dialog(
       _struct, CefBrowserCppToC::Wrap(browser), mode, title.GetStruct(),
-      default_file_path.GetStruct(), accept_filtersList, selected_accept_filter,
+      default_file_path.GetStruct(), accept_filtersList,
       CefFileDialogCallbackCppToC::Wrap(callback));
 
   // Restore param:accept_filters; type: string_vec_byref_const