Adds Support for Command-Line on Windows (#3699)

* + cli support on windows

- removes the preprocessor macro that prevented arg
  parsing on Windows
- adds windows-cli.cpp as src for a wrapper exe
- adds flameshot-cli target into cmake when building
  on Windows
- updates README

* updates PR

+ support for unicode characters in cli args
+ additional clarification in README re: flameshot-cli
+ new workaround for spaces in _popen path; works with relative output paths

* fix EOL

* updated flameshot.exe path construction

avoids using the hard-coded length of flameshot-cli.exe

* fix whitespace (clang)

src/windows-cli.cpp

@@ -0,0 +1,69 @@
+#include <iostream>
+#include <windows.h>
+
+std::wstring joinArgs(int argc, wchar_t* argv[])
+{
+    std::wstring result;
+    for (int i = 1; i < argc; ++i) {
+        if (i > 1) {
+            result += L" ";
+        }
+        result += argv[i];
+    }
+    return result;
+}
+
+void CallFlameshot(const std::wstring args, bool wait)
+{
+    // generate full path for flameshot executable
+    wchar_t path[MAX_PATH];
+    int pathLength = GetModuleFileNameW(NULL, path, MAX_PATH);
+    std::wstring pathstring(path);
+
+    // Find the last backslash to isolate the filename
+    size_t lastBackslash = pathstring.find_last_of(L'\\');
+    std::wstring directory = (lastBackslash != std::wstring::npos)
+                               ? pathstring.substr(0, lastBackslash + 1)
+                               : L"";
+
+    // generate command string
+    // note: binary path placed within quotes in case of spaces in path
+    int cmdSize = 32 + sizeof(directory) + sizeof(args);
+    wchar_t* cmd = (wchar_t*)malloc(sizeof(wchar_t) * cmdSize);
+    swprintf(cmd,
+             cmdSize,
+             L"\"%s\\flameshot.exe\" %s",
+             directory.c_str(),
+             args.c_str());
+    // call subprocess
+    FILE* stream = _wpopen(cmd, L"r");
+    free(cmd);
+    if (wait) {
+        if (stream) {
+            const int MAX_BUFFER = 2048;
+            char buffer[MAX_BUFFER];
+            while (!feof(stream)) {
+                if (fgets(buffer, MAX_BUFFER, stream) != NULL) {
+                    std::cout << buffer;
+                }
+            }
+        }
+        _pclose(stream);
+    }
+    return;
+}
+
+// Console 'wrapper' for flameshot on windows
+int wmain(int argc, wchar_t* argv[])
+{
+    // if no args, do not wait for stdout
+    if (argc == 1) {
+        std::cout << "Starting flameshot in daemon mode" << std::endl;
+        CallFlameshot(L"", false);
+    } else {
+        std::wstring argString = joinArgs(argc, argv);
+        CallFlameshot(argString, true);
+    }
+    std::cout.flush();
+    return 0;
+}