GLMixer Code

Use codec UTF8 by default in String and translation.
(should support accents and non latin characters in path and messages).

Improved dialog for Memory policy

Implementation of user selection of memory usage policy for VideoFile.
GUI selection in user preference dialog and saving of glmixer
preferences of the memory usage policy, allowing to limit buffers into
the range [60MB - 500MB] for each video file.

Implementation of the maximum memory usage for buffers of a video file.
The MAX_VIDEO_PICTURE_QUEUE_SIZE is 480MB, MAX_PACKET_QUEUE_SIZE = 20MB.
The DEFAULT_MEMORY_USAGE_POLICY is to 50%, i.e. 250MB per video file.

fix compilation issue version libavcodec (av_frame_free)

Edited wiki page Downloads through web user interface.

Edited wiki page Compilation through web user interface.

Memory management cleanup.
Possible fix of memory gluttony in VideoFile.cpp.

linking with lib pthread (ignore if not found) for cases when libav are
compiled with pthread (ideal solution would be to test, but doesn't
really matter..)

Changed default layout to stack Mixing, layout and current source
toolboxes (reported issue for low resolution screens).