Add a GUI shortcuts editor (Issue #279)

Embed libxfce4uis XfceShortcutsEditor widget in the Preferences Dialog.
The shortcuts of ThunarWindow, ThunarStandardView, ThunarStatusBar and ThunarLauncher are exposed.
The remaining shortcuts were deemed insignificant compared to the additional complexity that adding them
would introduce.

ThunarApplication watches the accelerator map for changes. Any change leads to all ThunarWindows reconnecting
their accelerators to make sure that currently active windows can use the new accelerators even if an action
previously did not have a shortcut

MR !171
Issue #279