StyledText: process DPI change only once per caret

[HeikoKlare]

In a recent fix to the Caret, we found that StyledText processes a DPI change for the same caret multiple times (see eclipse-platform/eclipse.platform.swt#1847 (review)). We agreed on cleaning that up in a follow-up task (which this issue is stupposed to cover).

Here is an extract of the original comment:

[...] part of the issue addressed here seems to also be that StyledText::updateAndRefreshCarets calls the update functionality on the same caret multiple times. Isn't that somehow unexpected and unintended as well? I.e., shouldn't StyledText::updateAndRefreshCarets probably first collect all carets to update (removing duplicated) and then update them instead of calling the update on the same caret multiple times?