Don't override pam environment variables

When using pam_env users are allowed to provide environment variables
to SDDM.

This is often used to customize PATH like Debian does.
However we overwrite those environment variables with our own.

What happens on Debian is that we overwrite PATH with a less
complete version.

Make sure pam variables are not overridden.

Closes #256

[ChangeLog][Behavioral Change] Don't override pam environment
variables.
Merge pull request #257 from plfiorini/pamenv

Don't override pam environment variables
Set default icon theme from greeter theme configuration

Let greeter themes specify the icon theme they want.
Closes #260
Set XCURSOR_THEME according to greeter theme

Let greeter themes override cursor theme.
Closes #226
Add commit template
Update ChangeLog
Remove wiki reference from commit template
Load first theme in alphabetical order if default is not available

Some distributions ship themes in separate packages so it's not sure
whether the default greeter theme ("maui") will be available.

Instead of forcing downstreams to configure sddm.conf with their theme,
we are clever enough to use the first theme available in alphabetical
order.

[ChangeLog][Daemon][Display] Load the first greeter theme in
alphabetical if the default one is not available.

Issue: #239