feat: add a pref to open files from applications in current window

[SolaWing]

add a new pref to open file from finder in current window.

closes #680 #642

[SolaWing]

@qvacua thank you for you review.

this pref work on:

1. drop files on app icon in dock bar.
2. open files from finder, like open with ... or double click

this pref not work on:

1. drop files into vimr app window, this always open in current window, and may be a seperate entry api.(I haven't check the code yet, do behaviors need to be unified? )

so it's not just only works for drag drop. for me, I rarely use droping files, most time i just double click files in finder. so limit the tip to drop & drag is not accurate. if you have any more clarity tip, you can comment me.

[qvacua]

Ah, I missed number 2. You're right, sorry about that. What about the following:

• In code: Maybe something along the lines of "open file from finder in" since I think just "open file in" is misleading, e.g., I first thought your PR was about the "Open..." menu item and only after looking at "AppDelegate" I realized what it was about (at least the half of it 😀)
• In pref window: What if we keep the title, but add a info text below describing what you described above?

As for the drag&drop onto the window, I have to look at it, but anyhow, it can be done separately (if necessary).

[SolaWing]

maybe we can make tip same as macvim, though currently we only support each tab option

drop onto the window, and open in that window, seems like a reasonable behavior . may not need to configure

[qvacua]

The info text of MacVim looks good. Yeah, the additional configuration can be built separately, if needed (I don't need it, but maybe others 😀).

[SolaWing]

@qvacua var and names changes accordings to macvim tip