Add keyboard shortcut to open and save file

[Gold872]

When opening the app, it can be very repetitive to have to open the menu, then click open file, and then navigate to your file. It would be nice to have Ctrl + O show the open file dialog.

Alternatively, it can open the previously opened file at startup if the file still exists.

[calcmogul]

Ctrl + O to open the file dialog sounds good. CLI file open would be better for opening a file from last time.

[Awesomeplayer165]

Yes I agree with @calcmogul. $choreo fileName.chor would be nice

[shueja]

Ctrl-O shortcut added in #435, open last file added in #395. Closing this.