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
Use the SessionCommand from the config instead of the one from cmake

Closes #263
Namespace'd everything auth related
Fix HelperApp compilation
Added a fake AuthPrompt::response property getter to keep the MOC parser happy

Closes #208
Fixed a bunch of typos in the README