New features and bug fixes

This merge request is related to the issue #158 which is a request for plugin integration.

If accepted, some features are necessary for the proper functioning of the plugins.

Each commits is related to a feature or bug which some have their own merge request. Here is the list with an explanation:

Feature 2e58c5f6 : Add a GtkBox on top of the the scrollview which can permit to add widgets like a toolbar.

Feature b5c3c43f : Handle the control of the monitors for external plugins. In some case we don't want to monitor a directory, or not getting the parent directory of an image. For example, in the plugin eog-images-manager I retrieve images by date and some virtual folder contains only one image and so on the click of the folder we doesn't want have all the images of the parent directory.

Feature ea3273d1 : Set eog_scroll_view_get_image_coords public. In one of my plugin, eog-toolbar, I have some filter which use the selection tool and for that it is necessary to get the coords of the image.

Feature 6f1625f6 : Related to the merge request !76 (closed) which is about displaying thumbnails faster and without lag when scrolling large folders of images.

Bug 8d80e9b8 : Related to the merge request !42 which is about the thumbnails being queued to often.

Bug 5fa97319 : Related to the merge request !78 (merged) which is about the image disappearing when we rename it from the file-manager.

Bug cdc66a8c : Related to the merge request !79 which is about when adding an image with an exif orientation in a monitored directory it is not oriented.

Feature c84bfb60 : Feature to display the text under the thumbnails.

Bug ef76570a : Related to the merge request !81 (merged) which is about freeing the GFile's list when we re-open eog with the flag single-window.

Feature 79fad766 : This signal is almost similar to the signal eog_image_modified, except is doesn't reset the zoom.

Feature ff2b7196 : This feature permit to add buttons up and down when the gallery is at the left or right of the scrollview.

Feature 4a1acc40 : This commit add in the gsettings the control to choose the size of the thumbnails. The values are 90 (the default size), 173 and 256.

Please note that this features doesn't change the behavior of eog and respect his philosophy.