Text cached measures should be share across instances

[GMartigny]

In Text class, the _cachedMeasures prevents re-computing measures each times when text hasn't changed.

It could be a slight improvement to share this cache across all Text instances. Getters and setters should exist through a Text static property.

It's not important for the user to access the cache, but it should be possible to:

• Add new value
• Read value
• Remove (all) value(s)