Fix invalid characters with visual assist

When using a keyboard layout that requires alt-gr to type "{", VsVim would
convert the newline character to an invalid 0x8D character. This is fixed
by not applying the alt and shift modifiers, if both modifers are down at
the same time.

This fixes bug #1390 and #1008, which are both really the same. The bugs really forced you to turn off many features of visual assist, so it was quite bad. English keyboard layouts were unaffected.

I'm not really sure if the solution is the best, but I guess you could get a better idea of how to fix it from this.