Fixed position of quit confirmation dialogue.

It is now centred on the window from which the quit menu item was selected, rather than the centre of the screen.
Fixed positioning of ScriptWindow close confirmation dialogue.

It is now centred properly on the ScriptWindow which is being closed.

Fixes #751.
Merge pull request #819 from johnhaddon/discardChanges

Discard changes
Added useNameAsPlugName argument to CompoundDataPlug::addMembers().

Also added python bindings for CompoundDataPlug::fillCompoundData() and CompoundDataPlug::fillCompoundObject().
Gave Displays node parameter plugs more useful names.
Added dropdown menu for Displays node quantize parameters.
Improved Displays node UI.

- Displayed only the label in the header section.
- Added file browser for name widget.
- Added a widget for editing the label in the collapsible section.
- Greyed out widgets when the display is turned off.
- Added delete button, visible when hovering over a particular display.
- Stopped deriving from CompoundPlugValueWidget and overriding _footer() and used PlugLayout instead - one less thing to deal with in the ongoing refactoring of plug UIs.

Fixes #15.
Merge pull request #823 from johnhaddon/displays

Displays UI
Fixed parsing of command line arguments with spaces.

This was fixed before in cf805242b54519286de801a82dabda43410a00e1 and then broken again in d991ed87ff4273a824a11ac3969abe1066c7bb8f.

Also reintroduced ~/gaffer/apps to GAFFER_APP_PATHS, as this was broken by the same commit.
Added command-line representation of Op values in the UI.

This is implemented by providing a "summary" Metadata entry for the node which contains the command line, and then using a small _InfoButton() class to display it. The summary is displayed both as a tooltip and also in a popup window when the button is clicked.

Since the NodeGraph also implements tooltips using the "summary" Metadata, the summary would also be available there if we used Ops there. We should implement summaries for more node types and then consider reusing the InfoButton more generally.

Fixes #793.
Worked around python quoting bug in op command line display.
Merge pull request #825 from johnhaddon/opCommandLine

Displaying Op command line in UI