Add Apply option to preferences dialog

[p9malino26]

I have added an 'Apply' button to the preferences dialog to save the changes without quitting the dialog.

[itsmattkc]

Is there an actual use-case to wanting to "apply" Olive's preferences dialog?

[p9malino26]

It saves the user from having to repeatedly re-open Preferences to change the same setting if they are trying to trial different settings in order to see what works for them. Appearance is a key example.

[ThomasWilshaw]

Olive is about to undergo some largish code refactoring and I wanted to check on the status of old PRs before we do that. Do you plan to update/continue with the PR? Thanks

[p9malino26]

It's up to you/the maintainers on whether to merge this pull request. I think this feature is quite useful though.