[Qt] remove GUIUtil::getSaveFileName() default arguments #3238

• harmonize function with GUIUtil::getOpenFileName()
• also make PNG Image singular (grammar)

Automatic sanity-testing: FAILED BUILD/TEST, see http://jenkins.bluematt.me/pull-tester/4f7d496b82210bf3235c1f8b41bfa821bfbfc54f for binaries and test log.

This could happen for one of several reasons:

1. It chanages paths in makefile.linux-mingw or otherwise changes build scripts in a way that made them incompatible with the automated testing scripts (please tweak those patches in qa/pull-tester)
2. It adds/modifies tests which test network rules (thanks for doing that), which conflicts with a patch applied at test time
3. It does not build on either Linux i386 or Win32 (via MinGW cross compile)
4. The test suite fails on either Linux i386 or Win32
5. The block test-cases failed (lookup the first bNN identifier which failed in https://github.com/TheBlueMatt/test-scripts/blob/master/FullBlockTestGenerator.java)

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Seems unrelated:

<pre> tail: write error: Broken pipe tail: write error </pre>

@TheBlueMatt Can you take a look?

Just curious: why not add a default arguments to getOpenFileName, that'd be a smaller change?

IMHO it's too easy to rely on default args, so I thought removing them would be better here.

Meh, the default arguments also don't really make sense (why would you want to provide empty messages), except for the final output argument.