re-enable map screenshot preview, fix minizip SIGSEGV, fix #141, ref #163

[illwieckz]

• The map scanning options were in the code but was'nt displayed, due to a mistake when the splash screen code was removed (see Get rid of Splash screen #93, Cleanup, gettext template, deprecate gtk1 support #109).
• Also, since the code use an external minizip (see Goodbye Autotools, hello CMake and minizip #147, rely on external minizip dependency instead of the builtin zip #141), I got a bug which is on some minizip release, XQF got SIGSEGV on unzGetCurrentFileInfo when used with some NULL pointer parameters, so I just added useless real pointers and it works.
• I've also made some cleaning of unused code.

So, without this fix, if the "scan at startup" option is activated, XQF crash at startup even if the option was not visible in the preference window.