Make context title and summary changeable (#329)

* Make context title and summary changeable

Closes #30

* Fix checkboxes on context settings page

So...I am new to Qt, and I forgot that checkboxes can have a label.
Duh. Fixed.

* Put context settings in a different place

* Put ReplaceMessage and ReplaceVariable in Utilities

src/settings/settingsdialog.h

@@ -75,6 +75,7 @@ class SettingsDialog : public QDialog {
     Page_Playlist,
     Page_GlobalShortcuts,
     Page_Appearance,
+    Page_Context,
     Page_Notifications,
     Page_Transcoding,
     Page_Proxy,