implement open file dialog for linux

src/app_events.cpp

@@ -14,6 +14,9 @@ std::string win32_open_dir();
 void win32_show_cursor(bool visible);
 bool win32_clipboard_set_text(const std::string & s);
 std::string win32_clipboard_get_text();
+#elif __APPLE__
+#else
+#include <tinyfiledialogs.h>
 #endif
 
 
@@ -190,6 +193,9 @@ void App::pick_file(std::vector<std::string> types, std::function<void (std::str
     std::string path = win32_open_file(filter.c_str());
     if (!path.empty())
         callback(path);
+#else
+    if (auto p = tinyfd_openFileDialog("Open File", "", 0, nullptr, nullptr, false))
+        callback(p);
 #endif
 }