Add a event Spy for GtkFileChooser recent files

Summary:
GtkFileChooser/GtkRecentManager follows https://www.freedesktop.org/wiki/Specifications/desktop-bookmark-spec/ specification to store its recently accessed files.
The history is stored in $HOME/.local/share/recently-used.xbel

Adds an event spy for the xml file used by gtk following the example of the KRecentDocument EventSpy.

It allows to populate recently accessed files from apps using Gtk, including inkscape, gimp...

Test Plan:
Compile
killall -9 kactivitymanage
run your localy built version of kactivitymanagerd, for instance:
~/kde/usr/lib/x86_64-linux-gnu/libexec/kactivitymanagerd
Run a gtk app and open a file
The file access can be seen in kickoff/kicker history

Reviewers: Frameworks, ivan

Reviewed By: ivan

Subscribers: bruns, ngraham, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23112