Provide osdialog-like async calls as Cardinal plugin API

[falkTX]

Plugins can use osdialog to deal with file open/save dialogs.
Since we cannot use osdialog we need to replace those, but that means a new/replacement API is also needed.
We should try to get 1-2 plugins that load files to work with this new API.

Later on we will need to make a list of plugins that use osdialog, perhaps this can even be automated.

[falkTX]

Attempting this now.

The custom Cardinal APIs being:

#ifdef USING_CARDINAL_NOT_RACK
void async_dialog_message(const char* message);
void async_dialog_message(const char* message, std::function<void()> action);
void async_dialog_filebrowser(bool saving, const char* filename, std::function<void(char* path)> action);
#endif

cf Player plugin patching test:

diff --git a/src/PLAYER.cpp b/src/PLAYER.cpp
index 6e0a920..05c4e15 100644
--- a/src/PLAYER.cpp
+++ b/src/PLAYER.cpp
@@ -411,8 +411,18 @@ struct PLAYERItem : MenuItem {
        PLAYER *rm ;
        void onAction(const event::Action &e) override {
                
+#ifdef USING_CARDINAL_NOT_RACK
+               async_dialog_filebrowser(false, nullptr, [this](char* path) {
+                       pathSelected(path);
+               });
+#else
                //std::string dir = module->lastPath.empty() ? NULL : rack::string::directory(module->lastPath); /////////////////////////////////////////
                char *path = osdialog_file(OSDIALOG_OPEN, NULL, NULL, NULL);        //////////dir.c_str(),
+               pathSelected(path);
+#endif
+       }
+
+       void pathSelected(char* path) {
                if (path) {
                        rm->play = false;
                        rm->reload = true;

[falkTX]

Going to finalize on these 4, which should cover all the needs of osdialog:

// opens a file browser, startDir and title can be null
// action is always triggered on close (path can be null), must be freed if not null
void async_dialog_filebrowser(bool saving, const char* startDir, const char* title,
                              std::function<void(char* path)> action);

// opens a message dialog with only an "ok" button
void async_dialog_message(const char* message);

// opens a message dialog with "ok" and "cancel" buttons
// action is triggered if user presses "ok"
void async_dialog_message(const char* message, std::function<void()> action);

// opens a text input dialog, message and text can be null
// action is always triggered on close (newText can be null), must be freed if not null
void async_dialog_text_input(const char* message, const char* text, std::function<void(char* newText)> action);

We ignore the color picker option, not sure if modules even use it.
If you disagree, if you have ~1h to speak up.