*This patch addresses an issue I stumbled upon while working on rewriting the history implementation.*
Currently the `updateviewarea` UI event doesn't include the `location`, and in the event handler in viewer.js we instead reach into `PDFViewer` to retrieve it. Not only does this seem conceptually wrong, it also makes the `updateviewarea` event less useful in general since it's currently only possible to access the `location` if you have a reference to the `PDFViewer`.
To me, this simply appears to be an oversight in the implementation, hence this patch which adds `location` to the `updateviewarea` event. It also changes `location` to be a "private" property of `PDFViewer`.
This patch:
- Adds a couple of utility methods to `PDFViewer` to enable checking `presentationModeState` without cumbersome comparisons.
- Disentangles `PresentationMode` from `PDFHistory` and `HandTool`, by adding event listeners for `presentationmodechanged` to both of them.
- Adds a utility function to `PDFViewerApplication` for requesting PresentationMode.
- Prevents initialization of `PresentationMode` if the browser does not support the fullscreen API.
From reading IRC scrollback, it became clear that having to manually call `PDFFindController.resolveFirstPage` in order for the find functionality to work isn't particulary good. Hence this PR, which moves that code into `PDFViewer.setDocument` to make life easier for third-party implementations.
