Merge #13216: [Qt] implements concept for different disk sizes on intro

9d0e52834 implements different disk sizes for different networks on intro (marcoagner)

Pull request description:

  Fixes https://github.com/bitcoin/bitcoin/issues/13213.
  Mostly, I layed out the concept to open the PR for refinement and getting feedback if the approach is okay. Changes are expected.

  Two points:
  - The values for both new consts `TESTNET_BLOCK_CHAIN_SIZE` and `TESTNET_CHAIN_STATE_SIZE` is certainly not optimal; I just checked the size of my testnet3 related dirs and set them to little bit higher values. Which values should be used?
  - Should we do something like this to regtest? Or these "niceties" do not matter when on regtest?

  Thanks!

Tree-SHA512: 8ae87a29fa8356b899e7a823c76cde793d9126b4ee59554d7a2a8edb088fe42a19976b34c06c2fd4a98a727e1e4971dd983f42b6093ea6caa255b45004e22bb4

src/interfaces/node.h

@@ -52,6 +52,12 @@ public:
     //! Choose network parameters.
     virtual void selectParams(const std::string& network) = 0;
 
+    //! Get the (assumed) blockchain size.
+    virtual uint64_t getAssumedBlockchainSize() = 0;
+
+    //! Get the (assumed) chain state size.
+    virtual uint64_t getAssumedChainStateSize() = 0;
+
     //! Get network name.
     virtual std::string getNetwork() = 0;