Update Dutch translation

Thanks. Committed in r.2045

Hello,

I've given the Dutch translation a thorough review, got a few typos out of it and tried to iron out some things that I feel were inconsistent with other translations elsewhere. I've also tried to make some texts more fluent, and to adhere to Dutch spelling rules. These spelling rules gave me headaches because tied-together words are very common and tend to become really long, difficult to read. In some cases I've therefore decided to use hyphens to split up words that actually should be written in 1 word, in order to improve readability ('articulatie-eigenschappen' is the only correct way to write this, but to keep consistency among all words ending in 'eigenschappen' and to avoid words of 5+ syllables, I also wrote 'notenbalk-eigenschappen' which should actually be without hyphen according to the rules). Anyway, Dutch spelling is a hell.

Regarding the Dutch translation, I realized that making the translation with Qt Linguist can be fairly difficult because developer comments in the original english are fairly rare. Some English words can translate to multiple things and I regularly had to search in MuseScore itself what the correct context was in order to provide the correct translation. Also in the original files, I find that very often re-typing the same translations but then without capital letters or without spaces needed to be done. I don't know really how Qt Linguist works, but it might be a point of attention in a future version to make some guidelines for text in the interface (e.g. capitalization: every word, almost every word, only the first word, only for full sentences. ... ? Now it's a mix, it seems).

In Dutch itself there are some subtle differences between Dutch Dutch and Flemish Dutch. Being Flemish, I've always learned note names with do-re-mi, not C-D-E. Only in international professional circuits, and for chord instruments like guitars, C-D-E is also in use in Flanders, but in education for other music instruments hardly any non-professional will be able to easily make the conversion. For now I've left the standard letter notation as is, I don't immediately know a solution (Doesn't the same problem arise in Germany, where H is used?).

The file in attachment has not been 'permanently approved', all things I've changed are still marked to be reviewed in Qt Linguist. Perhaps Jaap, you could take a look and let me know what you think of the changes I made? Of course feel free to revert some of these changes. I've tried as much as possible to add a comment to explain what I changed or why I changed things, but I might have lost some of my concentration after the 1000th item. Anyway, most changes are only very superficial (correction of capitalization, excess spaces, hyphenation).

Hope to be of help with this,

Joachim