Fix warnings

[NathanBnm]

I fixed some warnings in this PR but there are still some others. You should use css for font and background-color. There is also the Gtk.Alignement deprecation but I'm not sure about how to fix it so I didn't handle it.

../src/InputView.vala:70.25-70.65: warning: `Gtk.StyleContext.get_font' has been deprecated since 3.8
../src/InputView.vala:72.9-72.33: warning: `Gtk.Widget.override_font' has been deprecated since 3.16
../src/InputView.vala:245.29-245.41: warning: `Gtk.Alignment' has been deprecated since 3.14
../src/InputView.vala:245.13-245.21: warning: `Gtk.Alignment' has been deprecated since 3.14
../src/InputView.vala:251.13-251.42: warning: `Gtk.Widget.override_background_color' has been deprecated since 3.16
../src/ResultView.vala:40.27-40.39: warning: `Gtk.Alignment' has been deprecated since 3.14
../src/ResultView.vala:55.25-55.63: warning: `Gtk.StyleContext.get_font' has been deprecated since 3.8
../src/ResultView.vala:57.9-57.31: warning: `Gtk.Widget.override_font' has been deprecated since 3.16
../src/ResultView.vala:64.9-64.43: warning: `Gtk.Widget.override_background_color' has been deprecated since 3.16
../src/ResultView.vala:124.29-124.41: warning: `Gtk.Alignment' has been deprecated since 3.14
../src/ResultView.vala:124.13-124.21: warning: `Gtk.Alignment' has been deprecated since 3.14
../src/ResultView.vala:127.9-127.38: warning: `Gtk.Widget.override_background_color' has been deprecated since 3.16
../src/PasteBinDialog.vala:179.33-179.41: warning: `Gtk.Alignment' has been deprecated since 3.14
../src/PasteBinDialog.vala:179.17-179.25: warning: `Gtk.Alignment' has been deprecated since 3.14
../src/Tutorial.vala:45.9-45.27: warning: `Gtk.Button.xalign' has been deprecated since 3.14

[parnoldx]

Thank you