Bind the 'fnt' library to surge for string formatting (#4975)

https://github.com/fmtlib/fmt

Thanks!

.gitmodules

@@ -25,3 +25,6 @@
 [submodule "libs/LuaJitLib/LuaJIT"]
 	path = libs/LuaJitLib/LuaJIT
 	url = https://github.com/LuaJIT/LuaJIT.git
+[submodule "libs/fmt"]
+	path = libs/fmt
+	url = https://github.com/fmtlib/fmt.git

CMakeLists.txt

@@ -167,6 +167,7 @@ add_subdirectory(libs/airwindows)
 add_subdirectory(libs/catch2)
 add_subdirectory(libs/eurorack)
 add_subdirectory(libs/filesystem)
+add_subdirectory(libs/fmt)
 add_subdirectory(libs/tinyxml)
 add_subdirectory(libs/oddsound-mts)
 if(MINGW)
@@ -203,6 +204,7 @@ target_link_libraries(surge-shared PUBLIC
   samplerate
   surge-shared-binary
   tuning-library
+  fmt
 )
 
 # We want to run this once alas, since JUCE needs it even though it is a byproduct of a phase to build the

src/gui/SurgeGUIEditor.cpp

@@ -31,6 +31,7 @@
 #include "SurgeSynthEditor.h"
 
 #include "SurgeGUIEditorTags.h"
+#include "fmt/core.h"
 
 #include "overlays/AboutScreen.h"
 #include "overlays/CoveringMessageOverlay.h"
@@ -2380,7 +2381,7 @@ juce::PopupMenu SurgeGUIEditor::makeTuningMenu(const juce::Point<int> &where, bo
                               [this](const std::string &s) {
                                   float freq = ::atof(s.c_str());
                                   auto kb = Tunings::tuneA69To(freq);
-                                  kb.name = "Note 69 Retuned 440 to " + std::to_string(freq);
+                                  kb.name = fmt::format("Note 69 Retuned 440 to {:.2f}", freq);
                                   if (!this->synth->storage.remapToKeyboard(kb))
                                   {
                                       synth->storage.reportError("This .kbm file is not valid!",