### Problem description
Currently, the providers use the method `isSavable()` to determine both
if they can use "Save" or "Save as".
This behaviour is problematic because some providers may need to be
saveable but not saveable as: for example the view provider. The
original provider may not allow to be saved.
### Implementation description
I separate these two behaviour by creating another function:
`isDumpable()`, that return true by default but can be overridden by the
provider to return false, if the provider should not be dumped in any
way.
### Additional things
While I was at it, I also marked "export" operations as needing the
"dumpable" flag. That way, we can't accidentally export the whole
address space of a process as base64.
I also added documentation for these some functions in Provider
