Migrate from QString to QStringLists for settings, UI elements for hard-burn of subtitles (font, background)

[philstopford]

This PR started with adding UI elements to configure hard-burning of subtitles. In the process of making this work (with some notes below), I discovered why the tool was failing with inputs containing spaces and went ahead and fixed that.

The space race:

The tool previously compiled a string representation of all arguments that were to be passed to ffmpeg. This required careful tracking of trailing spaces, etc. and eventually split(" ") was called to break the string into a list of arguments. Naturally, if there were needed-spaces (e.g. -i /my path/to myfile.srt), this would break. It was also very fragile if multiple spaces were presented (" ") because the act of splitting would cause arguments to have empty entries and the invocation of ffmpeg would then fail To fix this up, I went through and moved everything to QStringLists. This gives natural separation because you have one argument or value per list entry. This also makes it easy to clean up empty slots (RemoveAll(" ")) and makes it also trivial to compile a string representation using .join(" ").

I've left some string/split usage in place where it was dealing with hard-coded stuff (e.g. PRESETS), but that may be nice to migrate at some point.

In testing, this now reliably works with subtitles, including hard-burn, where the path contains spaces. Very exciting. This was only tested on Linux.

There are remaining issues:

• For the life of me, I cannot get the QColorDialog to show alpha inputs. This is particularly useful for background for subtitles, to affect opacity. I could use a separate control, but it had appealed to me to see the visual effect. The KF libraries offer extended dialogs, but those would seem to pull in lots of KDE dependencies that are probably not ideal.
• Fonts with spaces in their names may cause ffmpeg to also throw a fit - this is yet to be tested again following the move to QStringLists.
• The UI code is pretty raw. It took me a fair bit of poking around to get this working and it's probably suboptimal or I missed things.
• The 'need to restart' is kind of annoying. I like the idea of having these in the global preferences because, for me, I tend to have a fixed style of hard-burning, and remembering to set it up in each preset could be tedious. At the same time, I yield to the project owner - if this ought to go via presets, I'm cool with that.

I also plan to add support for defining the position of the subtitles based on a fixed set of choices. It will be the focus of the next iteration of this WIP.

[philstopford]

Subtitle location is now, I hope, supported. I've also refactored a lot of the encoder code because the function was very long and difficult to navigate. Each 'module' is now broken out into its own method to reduce the confusion factor.

This probably needs a good amount of testing to make sure I didn't screw something up.

[philstopford]

I should note that fonts with spaces also work with this. One minor thing that I'd like to understand/have help with is to get the main application font (e.g. Noto Sans in my case) assigned as the default subtitle font absent user change. At the moment, this isn't happening and I think I'm missing something obvious/silly.