kernel: Add progress method to notifications

This commit is part of the libbitcoinkernel project and seeks to remove
the ChainstateManager's and, more generally, the kernel library's
dependency on interface_ui with options methods in this and the
following few commits. By removing interface_ui from the kernel library,
its dependency on boost is reduced to just boost::multi_index.

src/kernel/notifications_interface.h

@@ -6,9 +6,11 @@
 #define BITCOIN_KERNEL_NOTIFICATIONS_INTERFACE_H
 
 #include <cstdint>
+#include <string>
 
 class CBlockIndex;
 enum class SynchronizationState;
+struct bilingual_str;
 
 namespace kernel {
 
@@ -23,6 +25,7 @@ public:
 
     virtual void blockTip(SynchronizationState state, CBlockIndex& index) {}
     virtual void headerTip(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) {}
+    virtual void progress(const bilingual_str& title, int progress_percent, bool resume_possible) {}
 };
 } // namespace kernel