Reverts two commits:
977cd530c7bb9551de93900170bdaec4601b1b5b
820b7e432b57cd08c49d2e76cce4cb78016f0418
These break build from source on Linux for two reasons:
1.) The systemd user service needs to be installed in the `share`
prefix, not the `lib` prefix. This lets it get picked up in `~/.local`
but is also correct for just standard FHS paths.
2.) The `ghostty` path in the systemd user service needs to be absolute.
We should interpolate in the build install prefix to form an absolute
path.
The app.desktop file shipped with the default build presumes the install
location of the binary. Let the OS determine the executable through it's
normal PATH variables.
Inspired by #413.
This modifies the .desktop files so that it's possible to right-click on
the Ghostty icon in a dock and get the "New window" action.
Apparently this is how you implement that in GTK applications. In order
for us to also get "New tab", we'd have to implement a CLI flag, I
think.
