Issue #13812 - allow to override styles

This patch tries to address issue #13812 (performance of styled subs
with raspberry pi) via optional style overriding. Libass offers this
feature (used for example in vlc), via the ass_set_style_overrides
API.

The patch does the following :
- add a <libass> setting in advancedsettings.xml file with the
following structure :
   <libass>
    <styleoverrides>
        <style name="Shadow" value="0" />
        <style name="Blur" value="0" />
        <style name="BorderStyle" value="0" />
    </styleoverrides>
  </libass>
- add the corresponding string container in AdvancedSettings class
(values are converted to « name=value » format during parsing, which
is better than storing a map because it is what libass expects)
- calls ass_set_style_overrides at libass initialization with
the given parameters

Tests on raspberry pi 512 have shown that disabling effects such as
shadow and blur actually do suppress lags.