Being able to "stick" a page on a previous-view monitor

This patch allows the teacher to choose which page to use as a base for previous views.

In a multi-monitor Open-Sankoré environment, additional monitors will show the pages preceding the one the teacher is currently using (i.e. "previous views"). When only one additional monitor is available, it will show the previous page, and if the teacher wants to show another page, he's forced to change his current page too. This patch allows the teacher to explicitly choose, or "stick", a page as the one to show in the additional monitor, regardless of which page the teacher is working on. If more additional monitors are available, they will show the pages preceding the stuck one.

This patch adds a button in the page thumbnail widget, that will appear among the others (duplicate, delete, move up, move down) when the mouse hovers a thumbnail. The button has the icon of an external monitor, as used elsewhere in the app. By clicking on this button, the first additional monitor will show that page. The button remains visible even when the mouse is elsewhere, in order to inform the teacher that the previous views are stuck on that page. Sticking another page will un-stick the old page and stick the new one. "Re-sticking" the same page will disable sticking and the additional monitors will behave as before, i.e. showing the previous pages.