Allow change of output path

[dz0ny]

Adds new UI elements that allow to change and preview output path.

Does #11

[avsaase]

Hi, thank for the PR! I will review in in the next few days.

[avsaase]

I took a closer look at this and I'm not sure I like how this looks UI-wise. The empty or filled circles next to the input files are meant to indicate if the respective files are loaded but this doesn't make sense for the output path. Maybe adding a output path text box at the bottom of the screen similar to what Gyroflow has would be better?

There are also some other issues:

• The default width of the side bar is not large enough to fit the output file name.
• The output file name is formatted as a link but this doesn't work if the file doesn't exist. If it does exist for me it opens the video in my browser. I would rather have a button that opes the system's file explorer focused on the out put file.
• Saving the output path to the settings file should be optional, or at least the output path should be configurable. I organize my flight videos in folders named with the date of the flight so personally I want the videos to be rendered in the same folder as the input files.
• When there is no output path set in the saved settings file and you load a video, the path is not shown. It is saved to the file so when the app restarts it is shown, giving an inconsistent experience.

You're more than welcome to address these issues but I want to let you know that I'm considering switching from egui to Iced for the GUI. The first step is properly decoupling the UI and backend code and moving it into separate crates. This is already a pretty big diff and once I get to changing the UI code it'll be much bigger still. Merging in your changes later might become tricky and perhaps not worth it if the UI is redesigned anyway. I'll leave that decision up to you.