Imporvement of performance : Reduce the Java running count (3rd trial)

# What is it
- #6465 is the first trial.  So I followed up the albert advice.
	- in case of debug you use printf statements, I think that here there better options would be
		- create an -d flag (e.g. -d plantuml) that gives the information, so no need for using the configuration option
		- Follow up : create -d plantuml [debug.h, debug.cpp]
	- usage of printf
		- printf should not be used,all output is steered over calls to routines as defined in message.cpp,
		- Follow up : use Debug::Print(Debug::Plantuml, ...    and msg()
	- usage of std::string / std::map
		- in doxygen the string manipulation is. mostly, done by means of Qt classes (e.g. classes QCString, QString). For consistency these classes should be used.
		- map manipulation is also done by means of Qt classes (dictionary and list classes)
		- Follow up : use QDict QList QCString [plantuml.h, plantuml.cpp]
	- dirent / opendir /readdir
		- when browsing through directories the routines line readFileOrDirectory should probably be used or the techniques used in this routine
		- Follow up : I will follow when I try to reduce redundancy of creating plantuml image.
- #6467 is the second trial.  This is work items to satisfiy comments.
	- In the debug statements the __PRETTY_FUNCTION__ is a GCC extension. But this project can run in windows.
		- I got it.  I removed __PRETTY_FUNCTION__.
	- In the PLANTUML_RUN_JAVA_ONCE all the .pu files are given to the plantuml.jar with their full path (see -d extcmd when running). I think this will lead to problems when there are quite a few files ("command line" overflow). Seen this merging the files into one file like done for formulas. A small test with one file revealed that it didn't bring a lot / nothing time wise, but here the problem with the "command line" overflow shouldn't / can't happen.
		- Yes. The command line has overflow problem.  So I change into one file.
	- I think it still would make sense to implement the 'md5' possibility for, especially, the non PLANTUML_RUN_JAVA_ONCE version, i.e. if the file already exists don't try to recreate the image.
		- This is a next item.
			- PlantumlManager::instance()->insert() : this function will be changed to skip image.

# Configuration
- add configuration value in Doxyfile
	- PLANTUML_RUN_JAVA_ONCE = YES

# Debugging
- doxygen -d plantuml
- doxygen -d time -d extcmd

# Tested with following options
- source code of 3 plantuml
- PLANTUML_RUN_JAVA_ONCE = YES  or    NO
- DOT_IMAGE_FORMAT       = png  or    svg
- DOT_CLEANUP            = YES  or    NO
- -d plantuml
- -d time
- -d extcmd

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