Merge branch 'feature/granjow/decimal-separator'
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 Closes #713