Cache NPatch textures

- introduce NPatchLoader, a simple cache to store loaded N patches, their textures and metadata
- refactor NPatchVisual to use NPatchLoader and removed some dead / inefficient code

GL stat improvements in dali-demo

Before (only relevant numbers presented)
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INFO: DALI: OpenGL ES statistics sampled over 60 frames) operations per frame:
INFO: DALI:   Bind textures : Mean 109.17  (Min:107.00, Max:113.00, StdDev: 1.33, Actual:6550)
INFO: DALI: OpenGL ES Object Count:
INFO: DALI:   Texture Count : 101  (Peak:101)

After
-----------------------------------------
INFO: DALI: OpenGL ES statistics sampled over 60 frames) operations per frame:
INFO: DALI:   Bind textures : Mean 14.83  (Min:14.00, Max:15.00, StdDev: 0.37, Actual:890)
INFO: DALI: OpenGL ES Object Count:
INFO: DALI:   Texture Count : 8  (Peak:8)

Change-Id: I4ce84e6c6578c30660106950b40dd3bc0c41efa1