Fix compatibility with multisite

Changes:
- Bug fix: network activated plugins were not recognized (at all) and deactivating one would throw PHP notices.
- Bug fix: network activation action was not correctly unset if dependencies were not met (WP 3.4+)
- Show dependencies in the network admin plugins page
- Improved logic for recognizing whether dependencies have been satisfied
- Usability: On single site plugin page: added a "network" textual indicator for dependencies which were met by a network activated plugin
- Usability: On single site plugin page: made whether or not the plugin name is a link for network activated plugins dependent on whether the user can view the network admin plugin page as network activated plugins are not shown on the single site plugin page. Also pointed the link to the network page, rather than the single plugin page.
- Usability: If a network activated plugin (potentially) causes a site activated plugin to be deactivated, a transient will be set and shown as an admin notice to the site admin the first time they visit the plugins page after the deactivation.
If several such events have happened before the site admin visits the plugins page, all messages are saved and shown at once.
_**Note**_: This also means that site that site admins might see notices about 'deactivated' plugins which they didn't have activated on their site at all, but I figures that was preferable over not notifying single site admins at all.
- Minor textual changes to support the above (PR includes updated .pot file and updated Dutch translation)