Bugfix: Revised/repaired SFML2 sample

Changed the code such that the SFML 2 sample is compatible with the
revised libraries of SFML2
Bugfix: fixed error building libRocket with MinGW

When librocket was build with MinGW the wrong platform was selected. As
a consequence the wrong code was compiled which lead to corrupted
libraries.
Revert "Bugfix: fixed error building libRocket with MinGW"

This reverts commit 66b4d56779e6c5d370298d9f1ec47f56adf8003b.
Bugfix: fixed error building libRocket with MinGW

When librocket was build with MinGW the wrong platform was selected. As
a consequence the wrong code was compiled which lead to corrupted
libraries.
Revert "Bugfix: fixed error building libRocket with MinGW"

This reverts commit 773afa3a051f9907edce387f0d66142373f098cb.
Added a method to Factory which allows the template cache to be cleared.
This is similar to what we got for StyleSheetCache.
Renamed TemplateCache::ClearTemplateCache to TemplateCache::Clear
added REM unit support
Warning fixes.
Fix gcc compilation warnings

Mostly 'converting to non-pointer type int from NULL' and vice versa
Move ElementStyleCache.* to Core
More fixes for compilation warnings
Do not precalculate kerning for all characters

Precalculating kerning for all character pairs consumed a lot of RAM (N*N complexity). For CJK fonts that contain tens thousands of glyphs it was a highly unpractical approach resulting libRocket consuming >1.5GB of RAM. Instead, encapsulate FT_Get_Kerning call in FontFaceHandle::GetKerning and let FreeType handle the kerning on the fly. In my measurements, the performance difference is negligible, especially when compiled compiled geometry is utilized.
Fixed memory leaks and copy paste error in ElementTabSet
Fix pointer conversion bug in Variant
Fix return value of Element::GetOffsetParent()
Fix crash caused by pointer convertsion in drag sample
Copy-paste fix for elements with max_length set.
Copy-paste fix for elements with max_length set.
added sdl2 render interface sample