Subtitle style editor: Various cosmetics.

Reduce the visibility of the member functions and variables, use better name when possible.
ISR: Add a mode that automatically chooses the rendering target for text subtitles.

When using this mode, SSA/ASS subtitles will be rendered on the video frame while other text subtitles will be rendered on the full window.

Fixes #4345.
Add an advanced option to control the number of recent files.

Those files are shown in the "Recent Files" menu. It is also the files for which a position is potentially saved.

Fixes #1407 and #2425.
Advanced option page: Fix double-clicking on non-boolean item.

It prevented the page to be redrawn.
Fix "remember last window position" when "remember last window size" is off.

The size was always restored at start-up when "remember last window position" was enabled after ec0601a05626039f19aa4d51234408c491c18f63.
Fix: Make "remember last window size" behavior when exiting playback consistent with its start-up behavior.

When "remember last window size" was enabled, the window position was always retained which was inconsistent with the start-up behavior. Now the position will be retained only if "remember last window position" is enabled.
Async Reader: Initialize a member variable in the constructor.

Fixes CID 966379.
CBaseDispatch: Remove duplicated sanity check.

Fixes CID 1224558.
BaseClasses/wxutil.cpp: Ensure freed memory can't be reused by mistake.

Fixes CID 1224550.
Remove dead conditional.

CID 1224560.
Fix uninitialized pointer fields.

CID 966482.
Use the proper type for `bool`.
Fix uninitialized scalar fields.

CID 752183 and CID 752184
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
Why "C:\tmp.srt"?
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...