Add opt-in visible-overlay laser mode setting

tests/laser_setting_test.cpp

@@ -0,0 +1,35 @@
+#include "laser_setting.hpp"
+#include <cassert>
+#include <cstdlib>
+#include <filesystem>
+#include <fstream>
+#include <string>
+#include <vector>
+
+int main() {
+    using namespace frameyap;
+    const auto pattern = (std::filesystem::temp_directory_path() / "frameyap-laser-test-XXXXXX").string();
+    std::vector<char> buffer(pattern.begin(), pattern.end());
+    buffer.push_back('\0');
+    const char* created = ::mkdtemp(buffer.data());
+    assert(created);
+    const std::filesystem::path root(created);
+    assert(::setenv("XDG_CONFIG_HOME", root.c_str(), 1) == 0);
+    assert(default_laser_settings_path() == root / "frameyap/lasers-anytime");
+    const auto path = default_laser_settings_path();
+    assert(!load_lasers_anytime(path));
+    assert(save_lasers_anytime(path, true));
+    assert(load_lasers_anytime(path));
+    assert((std::filesystem::status(path).permissions() &
+            (std::filesystem::perms::group_all | std::filesystem::perms::others_all)) == std::filesystem::perms::none);
+    assert(save_lasers_anytime(path, false));
+    assert(!load_lasers_anytime(path));
+    { std::ofstream out(path, std::ios::trunc); out << "anything else\n"; }
+    assert(!load_lasers_anytime(path));
+    const auto link = root / "link";
+    std::filesystem::create_symlink(path, link);
+    assert(!load_lasers_anytime(link));
+    assert(!save_lasers_anytime(link, true));
+    assert(!load_lasers_anytime(path));
+    std::filesystem::remove_all(root);
+}