org.cinnamon.recorder: Add a output path key

[xproot]

as of Cinnamon 6.2.9, the recorder/screencast tool (see, System Settings -> Keyboard -> Shortcuts -> Screenshots and Recording -> Toggle recording desktop (must restart Cinnamon)) has no documented way of setting an output folder. With "well known" folders available, these currently get sent to $HOME/Videos.

The current options in dconf are: file-extension, framerate, and pipeline

• file-extension's default value is webm, and its description mentions

The filename for recorded screencasts will be a unique filename based on the current date, and use this extension. It should be changed when recording to a different container format.

• framerate's default value is 15, it does not allow for any extra input.
• pipeline's default value is none, but the description mentions that if it is none, this will be used: videorate ! vp8enc quality=10 speed=2 threads=%T ! queue ! webmmux

It is clear to me that none of these options allow setting a output path without modifying the pipeline in unsupported ways.

Proposed change:

Add a dconf key that will represent the output path of the file, if its value may be none or invalid, XDG_VIDEOS_DIR or alternatively, $HOME shall be used. Otherwise, if there is a valid path set the output file will get saved to that directory.