Apply only changed properties instead of reapplying them all.

Each call to `cre.setIntProperty()` and `cre.setStringProperty()` currently leads to reapplying  all properties, even those unchanged. Judging from the code in https://github.com/koreader/crengine/blob/b994e572f07f7aa8e1c42bf1230e723041368324/crengine/src/lvdocview.cpp#L5670, this is not really necessary and we can apply only changed properties. This could save as a few CPU cycles and potentionally prevent issues like koreader/koreader#569 or koreader/koreader#1177.