Keep the Config Dirty After a Failed Write and Sweep a Write's Own Leftovers

Before this change a config whose temporary could not be written stayed
dirty, so the idle handler tried again; the last round cleared the flag
regardless, which lost a pending change to a transient failure. The
writer reports whether the config itself was written and the flag clears
only then, as it always did.

The Windows rename retry duplicated what WindowsSupport::rename already
does, retrying and moving an open destination aside, and multiplied its
error logging; it is gone. The file a refused rename moves aside is
named so the sweep can clear it after a crash in between, and every
successful atomic write sweeps leftovers of earlier writes to the same
target, so the caches and the smaller state files are covered too; the
name shapes are strict enough that nothing of the user's matches.

The lock wait defaults to a second, long against a critical section of
milliseconds and short against the GUI thread; a lock file that cannot
be opened says to check its owner and permissions; the test timing
bounds tolerate a loaded runner; and the helper's header includes what
its declarations use.

src/libslic3r/AppConfig.hpp

@@ -451,9 +451,10 @@ private:
 
 	// Preset for each machine
 	MachineSettingMap											m_printer_settings;
-	// Writes the assembled config text, and on Windows its checksum and a backup copy. `checksum_source`
+	// Writes the assembled config text, and on Windows its checksum and a backup copy; false when the
+// config itself could not be written, in which case the caller stays dirty and retries. `checksum_source`
 	// is the text load() will verify, which for the JSON config ends before the trailing newline.
-	void												write_config_file(const std::string &path, std::string body, const std::string &checksum_source);
+	bool												write_config_file(const std::string &path, std::string body, const std::string &checksum_source);
 
 	bool														m_dirty;
 	// Original version found in the ini file before it was overwritten