Deferred creation of plug presets submenu.

In case someone does something like registering a preset which makes calls to an outrageously slow database somewhere.
Merge pull request #1129 from johnhaddon/deferredPresetsMenu

Deferred creation of plug presets submenu.
Added a Refresh button to ImageReaders.
Merge pull request #1130 from andrewkaufman/imageCache

Added a Refresh button to ImageReaders
Removed deprecated Path.addFilter() and Path.removeFilter() methods.
Added tokenize() function to StringAlgo.h.
Optimised PathMatcher with new tokenizer.

This approximately halves the time taken to construct a large PathMatcher from paths in string form.
Removed unused typedef from PathMatcher.
Removed relative imports from PathFilter classes.
WIP 1
Got filter signalling working.
Got copy() and info() working, with new attribute methods.
Got children() working.
Implemented remove() and __delitem__. FileSystemPathTest passes!
Added RunTimeTyped registrations for all python Path types.
Added binding for Path::emitPathChanged().

ScenePathTest passes!
Fixed PathFilter::changedSignal() binding.
Implemented FileSystemPath.copy().

All path classes will have to define a proper copy method now the base class is in C++.
Implemented Path __delitem__ and __setitem__ for slices.
Fixed GafferTest.PathFilterTest.

It was using an empty directory, and therefore testing nothing much at all.