fix capitalisation of various strings in dialogs

to comply with
http://musescore.org/developers-handbook/design/user-interface-text-style-guide.
Also change "Spatium" and "Space" to "Staff space" (where user visible),
see also discussion at http://musescore.org/en/node/36891.
Merge pull request #1385 from Jojo-Schmitz/capitalization

Some more text fixes