application: Do not unconditionally enable edit-current in PREVIEW

While the action is only meant to be enabled in PREVIEW, it can only
be enabled for documents that can be edited. This logic is already
encoded in EditView and the window_mode was trampling over it.

https://bugzilla.gnome.org/show_bug.cgi?id=723491
documents: Let DocCommon children indicate whether they support editing

Create a new canEdit virtual method, which can be implemented by the
individual classes. This is better than littering the rest of the code
with source-specific code.

https://bugzilla.gnome.org/show_bug.cgi?id=723491