HeliBoard 2.1

Changes since 2.0:

• fix broken functional key layout for tablets
• only show language switch key when there is something to switch to (#897)
• make default colors for "all colors" setting random instead of gray (#903)
• allow customizing start lag for gestures during typing, by @devycarol (#882, #894)
• allow customizing currency keys (#353)
• reduce long-press time for shift -> caps lock, by @devycarol (#924, #923)
• extend superscript popups in number row and symbols layout, by @b02860de585071a2 (#902, #901)
• minor fixes and improvements