Automatically edit filter string when user is typing

[crumblingstatue]

Fixes #89

Requirements from #89

There are also a few points about when we want to avoid the behavior:

• When the user presses Ctrl or Shift so that we can implement Key bindings in the future
  • Allow shift, but not other modifiers.
    Shift is probably desirable for capitalizing letters, so we want to allow editing while shift is being held down.
• When the user is already in any input field
  This seems to be working.

Extra behavior added

• The filter string is cleared when a new directory is entered. I think this behavior is desirable because it's much less likely that the user wants to keep the current filter when entering a new directory, rather than wanting to either see the unfiltered contents, or type a new filter from scratch. We could possibly add a way to configure this behavior in the future though.

Credits

@aymey made the changes to satisfy the requirements of not activating the behavior. Sorry, your commits got lost because I did a rebase against the latest develop revision.

[crumblingstatue]

I cleaned up the code as much as I can, this is now ready for review.
On demand, I can rebase to clean up the history.

[fluxxcode]

Thank you both for implementing this!
No need to rebase, I will squash the PR anyway :)

Clearing the search buffer (self.search_value.clear();) is now actually twice in FileDialog::load_directory since it was already implemented with v0.4.0. The other call for this is right at the beginning of the function. Could you remove the first call? I think where you've implemented it now it makes more sense.
Unfortunately I can't mark the lines in the review because you haven't made any changes there ^^

[crumblingstatue]

Clearing the search buffer (self.search_value.clear();) is now actually twice in FileDialog::load_directory

Whoops, I missed that. I added a commit that removes my redundant clear.

[crumblingstatue]

Could you remove the first call? I think where you've implemented it now it makes more sense.

Ah crap, sorry, I missed that you wanted to keep my change. Lazy reading, lol.
Fixed.