possible fix #75

I've added 'optimize' option (by default true). If it options is set
then sources will be checked if they are required (with dependencies if
any). And if they are not required they will be dropped by using common
mechanism.

Please note that if this option enabled the following features (at the
    moment) will be unavailable:

* key request from context menu (from configuration interface it still works)
* notifications event if sources on which notification is not connected

I suppose this commit will increase performance in about 4-5 times.

sources/awesome-widget/plugin/awkeycache.h

@@ -21,11 +21,15 @@
 
 #include <QHash>
 #include <QString>
+#include <QVariant>
 
 
 namespace AWKeyCache
 {
 bool addKeyToCache(const QString type, const QString key = QString(""));
+QStringList getRequiredKeys(const QStringList &keys, const QStringList &bars,
+                            const QVariantMap &tooltip,
+                            const QStringList &allKeys);
 QHash<QString, QStringList> loadKeysFromCache();
 };