GH-1314 add UI for custom minecraft jar addition

Also changes the text of the jar mod addition button.
It should be clearer what it does and hopefully will not confuse
as many people.

application/GuiUtil.h

@@ -7,4 +7,5 @@ namespace GuiUtil
 QString uploadPaste(const QString &text, QWidget *parentWidget);
 void setClipboardText(const QString &text);
 QStringList BrowseForFiles(QString context, QString caption, QString filter, QString defaultPath, QWidget *parentWidget);
+QString BrowseForFile(QString context, QString caption, QString filter, QString defaultPath, QWidget *parentWidget);
 }