PropertiesDialog messes around with tag values

[Gabriel Ebner <ge@…>]

After editing the tag key, PropertiesEditor changes the values combobox to a list of possible values, based on the current data.

This leads to following behaviour:

1) You get interesting suggestions, esp. for tags like 'name', which you've got to delete first before entering a new value and that's quite a hassle.

2) When you've already edited the tag value, and then decide to change the key (say, to correct a typo), your input will simply be discarded.

I've attached a patch that simply keeps the current tag value. The list is still there, you just have to pick one instead of being offered the first possibility (I honestly don't know where you can make a sensible guess for a tag value except for 'oneway'.)