Blender Git Loki

File Browser: Add F2 shortcut to rename files

Previously, we used Ctrl+Click for renaming, but since that shortcut is
now consistently used to add items to the selection, we can't use that.
In other cases we switched to F2 now, so it makes sense for the File
Browser too.
Further, AFAIK renaming was only possible through the context menu,
which makes it hard to discover in the right click select keymap (have
to press W).

Note that I had to do some internal changes to ensure the context menu
always acts on the clicked/hovered item, while the shortcut operates on
the active item. William and I agreed that this is likely the behavior
expected by most users.