Silence useless warning in src/json/json_spirit_writer_template.h to make important warnings easier to see.

warning: typedef ‘Char_type’ locally defined but not used [-Wunused-local-typedefs]
misc small changes to polish after include cleanup
[Qt] remove GUIUtil::getSaveFileName() default arguments

- harmonize function with GUIUtil::getOpenFileName()
- also make PNG Image singular (grammar)
Merge pull request #2980 from wtogami/silenceunusedwarning

Silence useless warning in src/json/json_spirit_writer_template.h to mak...
Merge pull request #3235 from Diapolo/misc

misc small changes to polish after include cleanup
Merge pull request #3238 from Diapolo/guiutil

[Qt] remove GUIUtil::getSaveFileName() default arguments