mpv.conf editing

[sfan5]

this covers quite many feature requests at once, since this allows full control over mpv behaviours
e.g.:

• fuzzy subtitle matching
• custom subtitle codepage
• save-position-on-quit
• playback speed (to some degree)

(also adds a nice dialog with mpv-android version and mpv --version output)

[jeeb]

Thanks for improving the commits. I have briefly tested this and it looked good. While the version-getting is just crazy, it seems to work and get more stuff than the properties (and as you have said on IRC, the logging receival can be useful somewhere else, too).

The difference to the version code-wise that I tested is as follows:

diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index b2f6b45..68e1967 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -76,7 +76,7 @@
    <string name="pref_advanced_version">Version information</string>

    <string name="pref_advanced_mpvconf">Edit mpv.conf</string>
    <string name="pref_advanced_mpvconf_message">You can directly edit mpv\'s config here.\n\nImportant: mpv-android will overwrite some settings, so don\'t try to set e.g. \"dscale\" here.\nGenerally this applies to all settings that have a [-"native"-]{+\"native\"+} equivalent in the settings menu.</string>


    <!-- Strings indirectly related to Settings -->

Thus LGTM, no reason to keep this thing out.