Added a 'Terminal' command for directory nodes, to open a terminal there.

Very handy in my workflow, which generally involves having a terminal to run the program in.

Note that the terminal command is configured in the preferences, and can start with dollar ('$') to make it an environment variable look-up.
Made the 'Explore' command a directory command rather than a file command.

This makes sense, since we can't really "explore" a file.

Also set the icon of the action to GTK_STOCK_DIRECTORY, which makes sense too now.
Bumped version to 1.2.0.
Fixed confused command enumeration.

Added terminal and explore commands to repository context menu, this just makes sense.

Removed strange code that cut out the last path component in the explore command ... Weird.
Documentation updates.

Re-took a bunch of screenshots to properly reflect the current menu command layout.

Added text about the terminal command configuration entry widget.