makefile: run qmake from the editor-subdir

Some versions/installations of qmake seems to have issues with
running from a different director than the .pro file is located
in when compiling resources, giving an error like:

<path-to-rcc>: File does not exist 'editor.qrc'

So let's just be straight forward and run qmake from the editor
subdir to side-step this.
makefile: do not remove editor/Makefile twice