Small fixes in the plaintext/plaintextflow languages

[alexanderpann]

• fixes Add context-assistant buttons for common operations #187
• fixes Deal with the Darcula color scheme #530
• fixes CharacterMatrix throws null pointer #569
• fixes Change to vertical/horizontal when wrapped with a macro, makes the macro disappear #302
• fixes Null-strings break generator #286
• fixes Splitting a word inside a loop, "factors out" part of the split contents #192

@eugenschindler can someone of you guys review the PR?

[sergej-koscejev]

I'm worried that calling PlaintextPaster.getStringFromClipboard() in action update may slow down action updates in the IDE too much (e.g. if I copy a large text from somewhere that I don't even intend to paste into MPS at all). I think a separate method should be added to only check whether there's a string in the clipboard without trying to retrieve it.

Otherwise looks good.