Running CEFMessageLoop form OnIdle, which may pontentially starve CEF
messageloop when application is busy with other UI-operation.
Change it to timer to avoid the starvation.
Due to webview sample building directory is different from platforms and
compilers, we need to retrieve the `<wxwidgets-src>/help/` absolute path
from building directory.
On Mac OS X, it will trigger a crash issue when user close the window
(It also appear in other platforms, but the crash doesn't appear.)
The crash is caused by calling `CefShutdown` in wxApp::OnExit(). When
wx application running in wxApp::OnExit function, the webframe has been
deleted, we need to call `CefShutdown` in webframe `Close` period.
Running CEF message loop inside wxWebviewChromium internal will get
segument fault, while moving to client doesn't.
Reset to the previous client solution to avoid segument falut, since I
don't figure out the segument fault reason.
* Enable webview_chromium on Linux, webview_chromium runs normally,
but browser window is split out of wxWidget window now and we need to
host it in wxWidgets window later.
* Make cef messageloop processing in wx::ProcessIdle period, this way
works on windows and linux.
* Don't use multiple_message_loop on Windows.
