Build-plugin: Move the build commands to a model

Modify and access the commands through the model-view interface.

Simplify the access mechanism so that we only have:
"select target" opens the target selection dialog
"default target" builds the default target of the current active target-set
"build previous" builds the previous or opens the target/command selection if there is no previous target/command
"stop" for stopping a running build
"next error" for jumping to the error
"previous error" for jumping to the previous error