pull code from mumax/3/gui
add select element
prepare for multi-level attribute setting (style.bla.bla = ...)
don't crash if onupdate == nil
fix element race condition and add comments
add html progress element
add meter element
minor fix in showErr
remove debug prints
add OnAnyEvent, IntVaoue FloatValue
add img, number, page_test.go
Merge remote-tracking branch 'gui/master' into arne

Conflicts:
	Makefile
	make.bash
poach-in and use github.com/barnex/gui
gui console auto-scroll down
gui: return of the CLI history
log cleanup
gui: better cli box
log cleanup
gui: don't crash on invalid CLI input
mark v3.5.2 alpha