hidePasswords, preferences, UI improvements.

[ADDED]     hidePasswords to Mac app for hiding the passwords in the large passwords display, hold alt to reveal.
[ADDED]     Way to easily open the Mac app's preferences.
[IMPROVED]  Use transparent main window to improve the blur effect.
[ADDED]     Key equivalent coach marks when holding alt.
[FIXED]     Don't change sites table when site text field loses focus.
Update screen capture code to accurately capture the active display only.
Remove dialog & add import/export

[REMOVED]   Mac password Dialog.
[ADDED]     Mac sites import / export.
[FIXED]     Copying of hidden passwords.