Add support for OTP code generation on Linux as requested in #327

[Noettore]

I've added support for pass-otp extension on Linux as requested in #327.

The configdialog now has an option to activate OTP code generation that happens pressing the button in the menu bar after selecting an entry. If the entry password is an otp url the code is generated, shown in the mainwindow and copied in the clipboard, otherwise an error is shown.

On windows and apple the option in the configdialog is hidden as well as the button. In imitatepass the OtpGenerate method is currently empty. I've already writted an otp library but for now I do not know how to integrate it in imitatePass. Is there a way to get the password of an entry so that I can pass it to the library for the otp code generation?

[coveralls]

Coverage decreased (-0.08%) to 6.351% when pulling 85f3b3b880e031b9277e0e645d7d0aa2835f51f2 on Noettore:feature/otp into eea0c7d2273a3bfd761121d9b4c86c68d27dcb06 on IJHack:master.

[codecov[bot]]

Codecov Report

Merging #394 into master will decrease coverage by 0.04%. The diff coverage is 2.94%.

@@            Coverage Diff            @@
##           master    #394      +/-   ##
=========================================
- Coverage     6.5%   6.45%   -0.05%     
=========================================
  Files          39      39              
  Lines        2553    2571      +18     
=========================================
  Hits          166     166              
- Misses       2387    2405      +18

Impacted Files	Coverage Δ
src/mainwindow.h	0% <ø> (ø)	:arrow_up:
src/pass.h	50% <ø> (ø)	:arrow_up:
src/configdialog.h	0% <ø> (ø)	:arrow_up:
src/imitatepass.h	25% <ø> (ø)	:arrow_up:
src/realpass.h	100% <ø> (ø)	:arrow_up:
src/qtpasssettings.h	0% <ø> (ø)	:arrow_up:
src/mainwindow.cpp	0% <0%> (ø)	:arrow_up:
src/qtpasssettings.cpp	0.91% <0%> (-0.01%)	:arrow_down:
src/pass.cpp	2.6% <0%> (+0.06%)	:arrow_up:
src/imitatepass.cpp	0.47% <0%> (ø)	:arrow_up:
... and 11 more

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