support "action" targets

it should be possible to add arbitrary "action" targets to projects.
a possible use case would be "check" targets, but it may be possible to implement install/deploy targets via this mechanism as well.

these targets would have the following features:

• they should be able to depend on other such targets and on regular products
• they should be triggerable explicitly, or implicitly upon certain events, e.g., "all nodes with a certain priority processed" (see QBS-265)
• their core implementation would be imperative (js)
• they should be able to "react" to location, e.g., "run check only for all subprojects of the current directory"
• there should be qbs library functionality to facilitate writing them, e.g., traversing the project tree recursively

these "action plugins" would be typically singletons, which are instantiated for example in parent/top-level projects.