Caches accounts window, editor and view in startup.

Caches accounts window, editor and view in startup.

* src/hildon2/modest-accounts-window.[ch]
  (new method modest_accounts_window_pre_create): offer a way to precreate
  the accounts window before using it, to increase the effort done in
  prestart stage.
* src/widgets/modest-window-mgr.[ch]: added new virtual method
  create_caches. This is used for creating the window caches we need, instead
  of doing this in specific maemo code. In common implementation we load
  cached instances of view and editor.
* src/widgets/modest-hildon2-window-mgr.c: added implementation of
  create_caches that pre creates accounts window.
* src/modest-main.c: added a call to window mgr create_caches in startup
  process.