Add persistency options

This enables elements to always be visible in configured states.
For example, setting `timeline_persistency=paused,audio` will make timeline always visible when playback is paused, or when audio file is open.

Available states: paused, audio

Available options and their defaults:
```
timeline_persistency=
volume_persistency=
speed_persistency=
top_bar_persistency=
```

resolves #42

README.md

@@ -60,6 +60,8 @@ timeline_size_min_fullscreen=0
 timeline_size_max_fullscreen=60
 # same thing as calling toggle-progress command once on startup
 timeline_start_hidden=no
+# comma separated states when timeline should always be visible. available: paused, audio
+timeline_persistency=
 # timeline opacity
 timeline_opacity=0.8
 # top border of background color to help visually separate timeline from video
@@ -80,6 +82,7 @@ chapters_opacity=0.3
 volume=right
 volume_size=40
 volume_size_fullscreen=60
+volume_persistency=
 volume_opacity=0.8
 volume_border=1
 volume_step=1
@@ -89,6 +92,7 @@ volume_font_scale=1
 speed=no
 speed_size=46
 speed_size_fullscreen=68
+speed_persistency=
 speed_opacity=1
 speed_step=0.1
 speed_font_scale=1
@@ -104,6 +108,7 @@ menu_font_scale=1
 # top bar with window controls and media title shown only in no-border mode
 top_bar_size=40
 top_bar_size_fullscreen=46
+top_bar_persistency=
 top_bar_controls=yes
 top_bar_title=yes