Fix decimal separator issues when loading/saving projects

This MR includes the following changes for decimal point handling:

• MLT XML in project files is now always stored with "C" as LC_ALL, meaning that it will always use . as decimal separator. This means that new project files should not run into issues with , or another character as decimal separator.
• When loading an old project file which uses a different decimal separators, the values are converted:
  • Effect parameters are converted based on their type
  • Filter parameters are converted based on a list of known properties which need conversion
  • General properties in the document are also converted based on a list (e.g. all frame_rate properties)

Related: #78 (closed) Closes #713 (closed)