[FrameData] Avoid iterating keys()

Instead use iterators. Could be changed to using keyBegin or something like that potentially.

The destructor is called 400 times on plasmashell startup for me.
Before: 250,000ns
After: 110,000ns

Differential Revision: https://phabricator.kde.org/D4350

src/plasma/framesvg.cpp

@@ -48,8 +48,8 @@ static const int MAX_FRAME_SIZE = 100000;
 
 FrameData::~FrameData()
 {
-    foreach (FrameSvg *frame, references.keys()) {
+    for (auto it = references.constBegin(), end = references.constEnd(); it != end; ++it) {
-        frame->d->frames.remove(prefix);
+        it.key()->d->frames.remove(prefix);
     }
 }