Multislection : change position and font

Issue : none

- Apply the current text line font or position to all selected subtitles
corresponding line
- Add a context menu when "right click" on selected subtitles. This menu
allow to change font attribute of all lines of all selected subtitles.
(Italic, underline, color, font). Allow to delete selected subtitles.

Signed-off-by: juiice <decaen.julien@gmail.com>