Avoid dialog headings showing up as some serif font in the iOS app

Apparently the use of [UIFont systemFontOfSize:10] familyName] to get
a default font family name is a bad idea. Presumably the return value
from this, ".SF UI Text", is matched against the list of font family
names enumerated from the system. (The "SF" apparently stands for "San
Francisco".) That ".SF UI Text" is not among them, so maybe vcl
chooses some arbitrary other font instead that happens to be a serif
one? If we instead use "Helvetica", at least we get a sans-serif font,
even if it doesn't match the system UI font exactly.

Change-Id: I7ff39d8e7893ce3c27f3f12d227f87209bbc7952

vcl/ios/iosinst.cxx

@@ -107,7 +107,7 @@ public:
     virtual void UpdateSettings( AllSettings &rSettings ) override
     {
         // Clobber the UI fonts
-        vcl::Font aFont( OUString::fromUtf8( [[[UIFont systemFontOfSize:10] familyName] UTF8String] ), Size( 0, 10 ) );
+        vcl::Font aFont( "Helvetica", Size( 0, 10 ) );
 
         StyleSettings aStyleSet = rSettings.GetStyleSettings();
         aStyleSet.SetAppFont( aFont );