Add support for "additional application actions"

[J0J0]

As per the Desktop Entry Specification (since version 1.1), "entries of type Application can include one or more actions. An action represents an additional way to invoke the application."

These commits add support for displaying the supported actions (via the already existing --property option) and running a specific action from a desktop file (via the new --action option).

Some remarks:

• The specification makes it not entirely clear to me whether properties like Terminal from the main application should apply to the additional applications actions. However, this seems to be the sensible choice.

• Furthermore, it says that:

  It is not valid to have an action group for an action identifier not mentioned in the Actions key. Such an action group must be ignored by implementors.

  The current impementation does not fully honor this: Though such an action group is not listed by --property Actions, it can still be run via --action, as long as its identifier is known. A check could easily be added, however, it seems unnecessary to me and the current ability rather appears as a feature than a problem.

[mattalxndr]

Whoops, I should have checked the list before opening PR #66. I like your approach, with a separate Action class. @J0J0