Don't overwrite events unnecessarily when parsing

[Thalexander]

Fixes #453

Adds a date_parsed and time_parsed property to determine whether to overwrite existing data. This may not handle the phrase "for x minutes/hours" (and equivalent German) correctly, however this was already not correct and so I would consider it a separate issue, we will need to determine if the date changes when saying a length that goes over the current day, I kept the existing behaviour of wiping data for these cases. Does not implement guest parsing.

[jeremypw]

It seems a bit bizarre that there has to be special code for particular languages hard-coded in just to edit the title of an event. I haven't looked at the code in detail but is this really necessary?

[Thalexander]

It is necessary to convey whether a time or a date has been detected, if we were to handle this in ParsedEvent I believe the only real way of doing so would be to have two separate DateTimes and then we would need to change the languages to not assign a DateTime by default but only if parsed, and then this in turn might complicate interactions when multiple expressions are parsed.

[jeremypw]

@Thalexander I cannot seem to reproduce the issue in master so it is difficult to check this fixes it. Do you have a way of reproducing the issue? I have tried with both US and UK english locales.

Update - dont worry I have found a way of reproducing now.

[jeremypw]

I can confirm this fixes the issue.

[jeremypw]

You should make a suitable entry in the appdata for the next release.