Move the lower_app_id lookup logic completely in the image_load_icon
method and use it also when looking up the icon from the desktop files
as well as icon themes.
If there are multiple icon themes defined in the config option
'icon-theme' the module will try from left to right to find an icon.
The system default will always be added to this list.
When using additional format options in addition to {icon} the format is
separated into text before and text after the icon. Each of the texts is
displayed in a separate label one before and one after the image for the
icon.
The code updating the labels on changes used the wrong format strings
when updating the label after the icon.
When only the option 'on-click-right' was set and no other 'on-click'
option than the taskbar module wouldn't register for click events and
hence those events were handled by the generic AModule::on-click code.
This code would try to start a shell with the specified command, which
wouldn't make any sense in this circumstances.
The taskbar code falsely checked for the 'on-click-left' option instead
for the 'on-click-right' when deciding to register for click events.
