English text line with key shortcuts to Song Screen

Add a small text line with key shortcuts to Song Screen, toggled if the search bar is active with help to the extended search keywords added by another commit.

Output/Languages/English.xml

@@ -152,6 +152,8 @@
   <string name="TR_SCREENSONG_NUMSONGS">(%v songs)</string>
   <string name="TR_SCREENSONG_NUMSONG">(%v song)</string>
   <string name="TR_SCREENSONG_ARTIST_TITLE">%a - %t</string>
+  <string name="TR_SCREENSONG_HELPMESSAGE">A-Z = Jump to the letter, F3 = Search, CTRL+R = Random selection, CTRL+S = Start medley, CTRL+A = Start all random, CTRL+V = Start visible random, ESC = Back to main menu</string>
+  <string name="TR_SCREENSONG_HELPMESSAGESEARCH">A-Z = Search artist and title, prefixes for the search: a: = artist / t: = title / c: = creator / e: = edition / g: = genre / y: = year (e.g. 1999 or 1980-1989) / l: = language / al: = album / fi: = file name / fo: = folder name</string>
   <string name="TR_SCREENSING_PAUSE">Pause</string>
   <string name="TR_SCREENSING_BUTTON_CANCEL">Stop singing</string>
   <string name="TR_SCREENSING_BUTTON_CONTINUE">Continue</string>