impr: Separate the behaviour of being savable and being dumpable for provider (#1183)

### 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

plugins/windows/include/content/providers/process_memory_provider.hpp

@@ -27,6 +27,7 @@ namespace hex::plugin::windows {
         [[nodiscard]] bool isWritable() const override { return true; }
         [[nodiscard]] bool isResizable() const override { return false; }
         [[nodiscard]] bool isSavable() const override { return false; }
+        [[nodiscard]] bool isDumpable() const override { return false; }
 
         void read(u64 address, void *buffer, size_t size, bool) override { this->readRaw(address, buffer, size); }
         void write(u64 address, const void *buffer, size_t size) override { this->writeRaw(address, buffer, size); }