Specify library dependencies in library.properties

Specifying the library dependencies in the depends field of library.properties causes the Arduino Library Manager (Arduino IDE 1.8.10 and newer) to offer to install any missing dependencies during installation of this library. "arduino-cli lib install" will automatically install the dependencies (arduino-cli 0.7.0 and newer).

library.properties

@@ -7,5 +7,6 @@ paragraph=Requires HW SPI and Adafruit_GFX. Caution: these e-papers require 3.3V
 category=Display
 url=https://github.com/ZinggJM/GxEPD2
 architectures=*
+depends=Adafruit GFX Library
 #next line is for support of Particle development environment, ignored by Arduino IDE
 dependencies.Adafruit_GFX_RK=1.3.5