panel-config: Add border_width property

[ryanabx]

This commit adds a new property to the panel config, which allows users to specify a border for the panel.

The default is 0.0, and we don't expose this property in cosmic-settings.

Design approval pending :)

Example of a border_width of 1.0:

[mmstick]

Needs approval by @maria-komarova. Probably fine since it's not enabled by default.

[maria-komarova]

It's all right to add the property as long as it doesn't change the defaults.

[wash2]

There seems to be a bug relating to how the border is drawn, as it doesn't line up correctly when I've tested it.

[ryanabx]

There seems to be a bug relating to how the border is drawn, as it doesn't line up correctly when I've tested it.

Could be because I used the iced border_width property? Maybe that's an outer border instead of an inner border. If so, there might need to be some size adjustments to accommodate, but that might affect the size of applets? Maybe not, /shrug