QmlDesigner: Improve imagecache

Instead of coding some arguments to extraId(state) we provide now a
std::variant there extra arguments can be saved. Because it's a
std::variant it can be easlily extended by new structs. There is a new
synchronous interface too. It has an extra method for QIcon which saves
icons in an extra table. It would be even nicer if we would have a
mipmap image too. So we could do it asynchonously too but so far it works
only in the main thread.

Task-number: QDS-3579
Fixes: QDS-3584
Change-Id: If368d84d82308a91a5f4f037021e749c9ef868ed
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/sqlite/sqlitebasestatement.h

@@ -51,6 +51,8 @@ namespace Sqlite {
 class Database;
 class DatabaseBackend;
 
+enum class Type : char { Invalid, Integer, Float, Text, Blob, Null };
+
 class SQLITE_EXPORT BaseStatement
 {
 public:
@@ -65,6 +67,7 @@ public:
     void step() const;
     void reset() const;
 
+    Type fetchType(int column) const;
     int fetchIntValue(int column) const;
     long fetchLongValue(int column) const;
     long long fetchLongLongValue(int column) const;