Keeping the state of filterText and loadOptions when jumping in and out of focus + filterText mouse support

[AndreasPB]

Hi again,

3 feature requests:

1. Ability to keep filterText state when going in and out of focus
2. Same, but with the result of loadOptions
3. Ability to select the filterText with your mouse

I'm currently using svelte-select to make a search-as-you-type for a very slow API and could use the ability to save some API calls when clicking in and out of the search field (request 1 and 2 would really help)

A lot of our customers use their mouse for editing text, so they would be unable to edit their filterText the way it currently works (request 3) Use case: Customer makes a typo in the middle of a long text and wants to edit the middle using the mouse to select the place.

Is this already possible (I haven't been able to find it in the docs/code)?

I'll happily split these into multiple issues if that's preferable