Compile qmldump tool for new Qt versions

Right now the qmldump tool is compiled together with the debugging helpers. The user can explicitly compile them in the Qt Versions dialog, or is asked to do so the first time he starts a gdb debugging session. This is not intuitive for users who 'just want to have code completion'.

Instead, we should compile the debugging helpers either the first time a project is compiled with a specific Qt version, or a project is loaded with a specific Qt version / switches Qt versions. This should ideally happen in a separate thread.