Clean up tip image files to prevent libpng warnings

- Small popup page accessible from the main menu via Learn to Play.
- Lets the player read through the tips with 'Previous' and 'Next' buttons.
- Tips continue to be shown on the loading screen.
-> but there without the scrolling ability.

- Added two new hotkeys for quicker tip changing (item.prev and item.next)
-> set to the left and right arrows respectively by default.

- Responsible scripts are placed in gui/reference/tips/.
- The tip text files have been moved to gui/reference/tips/texts/.
- Tip image files have been moved to art/textures/ui/tips/.

- Added a series of new sprites (textures in art/textures/ui/tipdisplay/)
-> comprises a title decoration, a bullet point sprite, and a new button
style.

binaries/data/config/default.cfg

@@ -418,6 +418,10 @@ rewind = "Shift+Backspace"        ; If timewarp mode enabled, go back to earlier
 next = "Tab", "Alt+S"             ; Show the next tab
 prev = "Shift+Tab", "Alt+W"       ; Show the previous tab
 
+[hotkey.item]
+next = "RightArrow"               ; Show the next item of a list
+prev = "LeftArrow"                ; Show the previous item of a list
+
 [hotkey.text]    ; > GUI TEXTBOX HOTKEYS
 delete.left = "Ctrl+Backspace"    ; Delete word to the left of cursor
 delete.right = "Ctrl+Del"         ; Delete word to the right of cursor