Don’t hardcode the use of Alacritty

[Phundrak]

The issue

Alacritty is hard-coded both in the default config and in panel_launcher.py. This may cause issues for people who do not have Alacritty installed.

Possible solution

The user could be able to set a variable containing their preferred terminal emulator in a terminal, and newm could rely then on this variable.

[CRAG666]

my solution

[Pandademic]

@Phundrak - this is a default, and it's noted in the docs.
Anway, like @CRAG666 showed in his screenshots, it's a easily customizable thing.