Use search path for simple texture expressions

The path to the texture can be in any subdirectory of the root path
or within any subdirectory of the search path. The relative path to
the texture will be discovered automatically.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
The problem is we need a relative path to another path which is not supported by boost filepath. And we must use Qt functions which support this functionality. Not sure if is such a good idea to move it into SearchPaths in that case.