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