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.
use dpi units
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
Bring back PPI-based units
Add inches to line height
Merge pull request #110 from dougbinks/master

Warning fixes.
Merge pull request #99 from RazorDE/master

Bugfix: Revised/repaired SFML2 sample
Merge pull request #111 from viciious/fixwarnings

Fix gcc compilation warnings