v0.4.0 (6) Symbols keyboard and fixes (2021-07-09)

• Added symbols keyboard and implemented switching to it
• Fixed NullPointerException for onSinglePointerTouchEvent of null key
• Fixed key press colour change dependent on activity background
• Implemented abort on move outside keyboard (by adding 1 px gutter)
• Updated keyboard font to Stroke Input Keyboard v1.2.1
• Improved "switch to symbols" label aesthetics
• Reduced space bar height
• Reduced digit row key height
• Added fillColour attribute for the keyboard as a whole
• Lightened key fill and text colours slightly
• Made parameters/variables final where possible