Fix document switcher plugin with multiple splitted view.

Summary: In differential D20548 an issue with document switcher plugin and split views is described. The solution is to open/activate document instead of activating view (aka Sublime::View) as view are linked to area (aka Sublime::Area) and will show again in same area instead of current active area.

But the differential solution was trying to retrieve the document from activated view which is not the perfect way as it must handle Sublime::UrlDocument to have an URL for openDocument function.

This patch use the document controller (aka IDocumentController) instead of sublime components.

The plugin tracks a list of document (aka IDocument) and enables signals for open/activate/close actions. Every document is placed in list model via DocumentSwitcherItem class responsible of extracting text and icon from a document passed in its constructor.

Without dependency to sublime, the plugin structure is simplified by not taking care about active main window, active area, and views. Only documents are tracking independently of the way they are displayed in UI.

Differential Revision: https://phabricator.kde.org/D24428