Added new class pqSettingsDialog which is a settings dialog constructed
by scanning all proxies registered under the "options" group. Created a
new proxy ("options", "GeneralSettings") that is created during session
initialization by vtkSMParaViewPipelineController. This keeps track of
the general application settings for ParaView.

Added mechanism in pqProxyWidget to look a "hint" on the proxy which
says that the documentation should be used for labelling the widgets.
Using that for the GeneralSettings proxy to construct a verbose settings
panel. Currently, on pqIntVectorPropertyWidget respects that setting
when construting label on its own. Otherwise, it works only when the
pqPropertyWidget doesn't render the label on its own.

Change-Id: Ide52f736748ab7a234307ab0b801b008de4e5703