Add property to set distance field cache size in Text in Qt Quick

It is currently only possible to set the sizes of distance fields at an application-global level, which means that if you want to have text which is either drawn with a large point size or zoomed in really far, you will have to increase the cache size of all text and thus potentially wasting a lot of memory.

We should have a low-level property to do this on a per-item basis. This is especially important now that the same Text items will be used in 3D, since you will typically want a higher resolution distance field when they are part of a navigatable 3D scene.