Improvements to the browser context menu

Hi there,
I made a couple of changes to the context menu:
* Removed the extension row, it seemed unnecessary given that it's already displayed in the name row.
* Removed FileItem.getExtension() and the FileItem.imageSize property.
* Renamed the 'Weight' and 'Size' rows to 'Size' and 'Dimensions', also anglicised the filesize units from 'Ko' and 'Mo' to 'KB' and 'MB'.
* Shifted the size and dimensions QML code into components so they can be called only when necessary.
* Added some more checks to WindowFiles.qml to prevent displaying non-supported files inthe viewer.

Note: I didn't see the point of wrapping the QML Text elements in Rectangles, so I removed that. Made the alignment much nicer. If they should be kept as Rectangles, then just let me know and I'll change them back :)