Markers

Hi all,

I've just added 3 feature requests [1], which I've kept separate as they 
are quite independent,  but essentially aim at improving marker 
usability (IMHO of course!).

I'm not sure how many users actually use markers, but I've been using 
them more recently for longer / more complex pieces and do think they 
are quite helpful for moving around a piece, quickly locating sections 
and also quickly creating / activating 'loop regions' (especially if 
#357 gets implemented) :-)

Lorenzo

[1] https://sourceforge.net/p/rosegarden/feature-requests/535/; 
https://sourceforge.net/p/rosegarden/feature-requests/536/; 
https://sourceforge.net/p/rosegarden/feature-requests/537/