Prepare TGMPA_List_Table for addition of update-workflow

Most significant changes:
- added views: 'to install', 'update available', 'to activate'
- simplified the `_gather_plugin_data()` method by abstracting parts of it to separate methods
- reverted the change to the constructor arguments from PR #227 as that breaks things, instead overload the `get_table_classes()` method which archives the same effect as PR #227 intended
- added 'version' column and a method to generate the output for it
- made column to display dependent on the chosen view
- abstracted the plugin row actions to a separate method
- made the row actions always visible (was on hover), similar to the WP native plugins table
- added upgrade notice information to a plugin row if available

Author: jrfnl